Nanoparticles

ABSTRACT

Method for producing a nanoparticle comprised of core, first shell and second shell semiconductor materials. Effecting conversion of a core precursor composition comprising separate first and second precursor species to the core material and then depositing said first and second shells. The conversion is effected in the presence of a molecular cluster compound under conditions permitting seeding and growth of the nanoparticle core. Core/multishell nanoparticles in which at least two of the core, first shell and second shell materials incorporate ions from groups 12 and 15, 14 and 16, or 11, 13 and 16 of the periodic table. Core/multishell nanoparticles in which the second shell material incorporates at least two different group 12 ions and group 16 ions. Core/multishell nanoparticles in which at least one of the core, first and second semiconductor materials incorporates group 11, 13 and 16 ions and the other semiconductor material does not incorporate group 11, 13 and 16 ions.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/997,973, filed on Feb. 5, 2008, which is the U.S. national stageapplication of International (PCT) Patent Application Serial No.PCT/GB2006/003028, filed Aug. 14, 2006, which claims the benefit of GBApplication No. 0516598.0, filed Aug. 12, 2005. The entire disclosuresof these applications are hereby incorporated by reference as if setforth at length herein in their entirety.

BACKGROUND

There has been substantial interest in the preparation andcharacterisation of compound semiconductors comprising of particles withdimensions in the order of 2-100 nm, often referred to as quantum dotsand nanocrystals mainly because of their optical, electronic or chemicalproperties. These interests have occurred mainly due to theirsize-tuneable electronic, optical and chemical properties and the needfor the further miniaturization of both optical and electronic devicesthat now range from commercial applications as diverse as biologicallabelling, solar cells, catalysis, biological imaging, light-emittingdiodes amongst many new and emerging applications.

Although some earlier examples appear in the literature, recentlymethods have been developed from reproducible “bottom up” techniques,whereby particles are prepared atom-by-atom, i.e. from molecules toclusters to particles using “wet” chemical procedures. Rather from “topdown” techniques involving the milling of solids to finer and finerpowders.

To-date the most studied and prepared of nano-semiconductor materialshave been the chalcogenides II-VI materials namely ZnS, ZnSe, CdS, CdSe,CdTe; most noticeably CdSe due to its tuneability over the visibleregion of the spectrum. Semiconductor nanoparticles are of academic andcommercial interest due to their differing and unique properties fromthose of the same material, but in the macro crystalline bulk form. Twofundamental factors, both related to the size of the individualnanoparticle, are responsible for these unique properties.

The first is the large surface to volume ratio; as a particle becomessmaller, the ratio of the number of surface atoms to those in theinterior increases. This leads to the surface properties playing animportant role in the overall properties of the material.

The second factor is that, with semiconductor nanoparticles, there is achange in the electronic properties of the material with size, moreover,the band gap gradually becoming larger because of quantum confinementeffects as the size of the particles decreases. This effect is aconsequence of the confinement of an ‘electron in a box’ giving rise todiscrete energy levels similar to those observed in atoms and molecules,rather than a continuous band as in the corresponding bulk semiconductormaterial. For a semiconductor nanoparticle, because of the physicalparameters, the “electron and hole”, produced by the absorption ofelectromagnetic radiation, a photon, with energy greater then the firstexcitonic transition, are closer together than in the correspondingmacrocrystalline material, so that the Coulombic interaction cannot beneglected. This leads to a narrow bandwidth emission, which is dependentupon the particle size and composition. Thus, quantum dots have higherkinetic energy than the corresponding macrocrystalline material andconsequently the first excitonic transition (band gap) increases inenergy with decreasing particle diameter.

The coordination about the final inorganic surface atoms in any core,core-shell or core-multi shell nanoparticles is incomplete, with highlyreactive “dangling bonds” on the surface, which can lead to particleagglomeration. This problem is overcome by passivating (capping) the“bare” surface atoms with protecting organic groups. The capping orpassivating of particles not only prevents particle agglomeration fromoccurring, it also protects the particle from its surrounding chemicalenvironment, along with providing electronic stabilization (passivation)to the particles in the case of core material.

The capping agent usually takes the form of a Lewis base compoundcovalently bound to surface metal atoms of the outer most inorganiclayer of the particle, but more recently, so as to incorporate theparticle into a composite, an organic system or biological system cantake the form of, an organic polymer forming a sheaf around the particlewith chemical functional groups for further chemical synthesis, or anorganic group bonded directly to the surface of the particle withchemical functional groups for further chemical synthesis.

Single core nanoparticles, which consist of a single semiconductormaterial along with an outer organic passivating layer, tend to haverelatively low quantum efficiencies due to electron-hole recombinationoccurring at defects and dangling bonds situated on the nanoparticlesurface which lead to non-radiative electron-hole recombinations.

One method to eliminate defects and dangling bonds is to grow a secondmaterial, having a wider band-gap and small lattice mismatch with thecore material, epitaxially on the surface of the core particle, (e.g.another II-VI material) to produce a “core-shell particle”. Core-shellparticles separate any carriers confined in the core from surface statesthat would otherwise act as non-radiative recombination centres. Oneexample is ZnS grown on the surface of CdSe cores. The shell isgenerally a material with a wider bandgap then the core material andwith little lattice mismatch to that of the core material, so that theinterface between the two materials has as little lattice strain aspossible. Excessive strain can further result in defects andnon-radiative electron-hole recombination resulting in low quantumefficiencies.

Quantum Dot-Quantum Wells

Another approach which can further enhance the efficiencies ofsemiconductor nanoparticles is to prepare a core-multi shell structurewhere the “electron-hole” pair are completely confined to a single shellsuch as a quantum dot-quantum well structure. Here, the core is of awide bandgap material, followed by a thin shell of narrower bandgapmaterial, and capped with a further wide bandgap layer, such asCdS/HgS/CdS grown using a substitution of Hg for Cd on the surface ofthe core nanocrystal to deposit just a few monolayer of HgS. Theresulting structures exhibited clear confinement of photoexcitedcarriers in the HgS. Other known Quantum dot quantum well (QDQW)structures include—ZnS/CdSe/ZnS, CdS/CdSe/CdS and ZnS/CdS/ZnS.Colloidally grown QD-QW nanoparticles are relatively new. The first andhence most studied systems were of CdS/HgS/CdS grown by the substitutionof cadmium for mercury on the core surface to deposit one monolayer ofHgS. A wet chemical synthetic method for the preparation of sphericalCdS/HgS/CdS quantum wells was presented with a study of their uniqueoptical properties. The CdS/HgS/CdS particles emitted a red band-edgeemission originating from the HgS layer. Little et al. have grownZnS/CdS/ZnS QDQWs using a similar growth technique to that of Eychmüllerto show that these structure can be made despite the large latticemismatch (12%) between the two materials, ZnS and CdS. Daniels et alproduced a series of structures that include ZnS/CdSe/ZnS,ZnS/CdS/CdSe/ZnS, ZnS/CdSe/CdS/ZnS, ZnS/CdS/CdSe/CdS/ZnS. The aim ofthis work was to grow strained nanocrystalline heterostructures and tocorrelate their optical properties with modelling that suggested thatthere is relocation of the carriers (hole/electron) from confinement inthe ZnS core to the CdSe shell. CdS/CdSe/CdS QDQW's, have also beenproduced by Peng et al. although this structure is promising, the smallCdS band gap may not be sufficient to prevent the escape of electrons tothe surface.

Although there are now a number of methods for preparing core-shellquantum dots, where it has been shown and reported for the reactionsolutions containing the quantum dots, core-shell quantum dots can havequantum yields as high as 90%. However, it is well known that once onetries to manipulate the freshly made solutions of core-shell quantumdots such as isolating the particles as dry powders, upon re-dissolvingthe particles quantum yields can be substantially lower (sometimes aslow as 1-5%).

According to a first aspect of the present invention there is provided amethod for producing a nanoparticle comprised of a core comprising acore semiconductor material, a first layer comprising a firstsemiconductor material provided on said core and a second layercomprising a second semiconductor material provided on said first layer,said core semiconductor material being different to said firstsemiconductor material and said first semiconductor material beingdifferent to said second semiconductor material, wherein the methodcomprises effecting conversion of a nanoparticle core precursorcomposition to the material of the nanoparticle core, depositing saidfirst layer on said core and depositing said second layer on said firstlayer, said core precursor composition comprising a first precursorspecies containing a first ion to be incorporated into the growingnanoparticle core and a separate second precursor species containing asecond ion to be incorporated into the growing nanoparticle core, saidconversion being effected in the presence of a molecular clustercompound under conditions permitting seeding and growth of thenanoparticle core.

This aspect of the present invention relates to a method of producingcore/multishell nanoparticles of any desirable form and allows readyproduction of a monodisperse population of such particles which areconsequently of a high purity. It is envisaged that the invention issuitable for producing nanoparticles of any particular size, shape orchemical composition. A nanoparticle may have a size falling within therange 2-100 nm. A sub-class of nanoparticles of particular interest isthat relating to compound semiconductor particles, also known as quantumdots or nanocrystals.

The current invention concerns the large scale synthesis ofnanoparticles by the reaction whereby a seeding molecular cluster isplaced in a dispersing medium or solvent (coordinating or otherwise) inthe presence of other precursors to initiate particle growth. Theinvention uses a seeding molecular cluster as a template to initiateparticle growth from other precursors present within the reactionmedium. The molecular cluster to be used as the seeding agent can eitherbe prefabricated or produced in situ prior to acting as a seeding agent.

Although manipulation of freshly made solutions of core-shell quantumdots can substantially lower the particles' quantum yields, by using acore-multishell architecture rather than known core-shell structures,more stable nanoparticles (to both chemical environment and photoeffects) can be produced. It will be appreciated that while the firstaspect of the present invention defines a method for producingnanoparticles having a core, and first and second layers, the methodforming the first aspect of the present invention may be used to providenanoparticles comprising any desirable number of additional layers (e.g.third, fourth and fifth layers provides on the second, third and fourthlayers respectively) of pure or doped semiconductor materials, materialshaving a ternary or quaternary structure, alloyed materials, metallicmaterials or non-metallic materials. The invention addresses a number ofproblems, which include the difficulty of producing high efficiency blueemitting dots.

The nanoparticle core, first and second semiconductor materials may eachpossess any desirable number of ions of any desirable element from theperiodic table. Each of the core, first and second semiconductormaterial is preferably separately selected from the group consisting ofa semiconductor material incorporating ions from groups 12 and 15 of theperiodic table, a semiconductor material incorporating ions from groups13 and 15 of the periodic table, a semiconductor material incorporatingions from groups 12 and 16 of the periodic table, a semiconductormaterial incorporating ions from groups 14 and 16 of the periodic tableand a semiconductor material incorporating ions from groups 11, 13 and16 of the periodic table.

Thus, while at least one of the core, first and second semiconductormaterials may incorporate ions from groups 12 and 15 of the periodictable, the material(s) used in these layers may include ions of one ormore further elements, for example, more than one element from group 12and/or group 15 of the periodic table and/or ions from at least onedifferent group of the periodic table. A preferred core/multishellarchitecture comprises at least one layer incorporating two differenttypes of group 12 ions (e.g. Cd and Zn, or Cd and Hg) and group 16 ions(e.g. S, Se or Te).

In the nanoparticle of the present invention where at least one of thecore, first and second semiconductor materials is selected from thegroup consisting of a semiconductor material incorporating ions fromgroups 12 and 15 of the periodic table (a ‘II-V’ semiconductormaterial), a semiconductor material incorporating ions from groups 14and 16 of the periodic table (a ‘IV-VI’ semiconductor material) and asemiconductor material incorporating ions from groups 11, 13 and 16 ofthe periodic table (a ‘I-III-VI’ semiconductor material), any othercore, first or second layers in a particular nanoparticle may comprise aII-V, IV-VI or I-III-VI material. For example, where a nanoparticle inaccordance with the present invention has a core comprising a II-Vsemiconductor material, the nanoparticle may possess a first layercomprising any appropriate semiconductor material for example adifferent II-V material (i.e. a II-V material in which the II ions areions of a different element of group 12 compared to the II ions in thecore material and/or the V ions are ions of a different element comparedto the group 15 ions in the core material), or a IV-VI or I-III-VIsemiconductor material. Furthermore, if the nanoparticle in accordancewith the present invention possess a second layer comprising a I-III-VIsemiconductor material, it may possess a first layer comprising anysuitable semiconductor material including a different I-III-VIsemiconductor material, or a II-V or IV-VI material. It will beappreciated that when choosing suitable semiconductor materials to placenext to one another in a particular nanoparticle (e.g. when choosing asuitable first layer material for deposition on a core, or a suitablesecond layer material for deposition on a first layer) considerationshould be given to matching the crystal phase and lattice constants ofthe materials as closely as possible.

The method forming the first aspect of the present invention may be usedto produce a nanoparticle comprised of a core comprising a coresemiconductor material, a first layer comprising a first semiconductormaterial provided on said core and a second layer comprising a secondsemiconductor material provided on said first layer, said coresemiconductor material being different to said first semiconductormaterial and said first semiconductor material being different to saidsecond semiconductor material, wherein

a) at least two of the core, first shell and second shell materialsincorporate ions from groups 12 and 15 of the periodic table, groups 14and 16 of the periodic table, or groups 11, 13 and 16 of the periodictable;b) the second shell material incorporates ions of at least two differentelements from group 12 of the periodic table and ions from group 16 ofthe periodic table;c) at least one of the core, first and second semiconductor materialsincorporates ions from groups 11, 13 and 16 of the periodic table and atleast one other of the core, first and second semiconductor materials isa semiconductor material not incorporating ions from groups 11, 13 and16 of the periodic table.

Preferably in set a) the other of the core, first and secondsemiconductor materials incorporates ions from the group consistinggroups 12 and 15 of the periodic table, groups 13 and 15 of the periodictable, groups 12 and 16 of the periodic table, groups 14 and 16 of theperiodic table, and groups 11, 13 and 16 of the periodic table.

It is preferred that in set b) said second semiconductor material hasthe formula M_(x)N_(1-x)E, where M and N are the group 12 ions, E is thegroup 16 ion, and 0<x<1. It is preferred that 0.1<x<0.9, more preferably0.2<x<0.8, and most preferably 0.4<x<0.6. Particularly preferrednanoparticles have the structure ZnS/CdSe/Cd_(x)Zn_(1-x)S,Cd_(x)Zn_(1-x)S/CdSe/ZnS or Cd_(x)Zn_(1-x)S/CdSe/Cd_(x)Zn_(1-x)S.

In a preferred embodiment of set c) said at least one other of the core,first and second semiconductor materials not incorporating ions fromgroups 11, 13 and 16 of the periodic table incorporates ions from thegroup consisting of groups 12 and 15 of the periodic table, groups 13and 15 of the periodic table, groups 12 and 16 of the periodic table,and groups 14 and 16 of the periodic table.

Preferably the nanoparticle formed using the method according to thefirst aspect of the present invention further comprises a third layer ofa third semiconductor material provided on said second layer. Thenanoparticle may optionally comprise still further layers ofsemiconductor material, such as fourth, fifth, and sixth layers.

It is preferred that the third semiconductor material is selected fromthe group consisting of a semiconductor material incorporating ions fromgroups 12 and 15 of the periodic table, a semiconductor materialincorporating ions from groups 13 and 15 of the periodic table, asemiconductor material incorporating ions from groups 12 and 16 of theperiodic table, a semiconductor material incorporating ions from groups14 and 16 of the periodic table and a semiconductor materialincorporating ions from groups 11, 13 and 16 of the periodic table.

Preferably the group 12 ions are selected from the group consisting ofzinc ions, cadmium ions and mercury ions. The group 15 ions arepreferably selected from the group consisting of nitride ions, phosphideions, arsenide ions, and antimonide ions. It is preferred that the group14 ions are selected from the group consisting of lead ions, tin ionsand germanium ions. Preferably the group 16 ions are selected from thegroup consisting of sulfide ions, selenide ions and telluride ions. Thegroup 11 ions are preferably selected from the group consisting ofcopper ions, silver ions and gold ions. In a preferred embodiment thegroup 13 ions are selected from the group consisting of aluminium ions,indium ions and gallium ions.

The core, first and second semiconductor materials may include ions inan approximate 1:1 ratio (i.e. having a stoichiometry of 1:1). Forexample, the nanoparticle ZnS/CdTe/ZnS contains a first layer of CdTe inwhich the ratio of cadmium to telluride ions is approximately 1:1. Thesemiconductor materials may possess different stroichiometries, forexample the nanoparticle ZnS/CuInS₂/ZnS contains a first layer of CuInS₂in which the ratio of copper to indium ions is approximately 1:1 but theratio of copper to sulfide ions is 1:2 and the ratio of indium tosulfide ions is 1:2. Moreover, the semiconductor materials may possessnon-empirical stoichiometries. For example, the nanoparticleZnS/CuInS₂/Cd_(x)Zn_(1-x)S incorporates a second layer ofCd_(x)Zn_(1-x)S where 0<x<1. The notation M_(x)N_(1-x)E is used hereinto denote a mixture of ions M, N and E (e.g. M=Cd, N═Zn, E=S) containedin a semiconductor material. Where the notation M_(x)N_(1-x)E is used itis preferred that 0<x<1, preferably 0.1<x<0.9, more preferably0.2<x<0.8, and most preferably 0.4<x<0.6.

The temperature of the dispersing medium containing the growingnanoparticles may be increased at any appropriate rate depending uponthe nature of the nanoparticle core precursor composition and themolecular cluster compound being used. Preferably the temperature of thedispersing medium is increased at a rate in the range 0.05° C./min to 1°C./min, more preferably at a rate in the range 0.1° C./min to 1° C./min,and most preferably the temperature of the dispersing medium containingthe growing nanoparticles is increased at a rate of approximately 0.2°C./min.

Any suitable molar ratio of the molecular cluster compound to first andsecond nanoparticle core precursors may be used depending upon thestructure, size and composition of the nanoparticles being formed, aswell as the nature and concentration of the other reagents, such as thenanoparticle core precursor(s), capping agent, size-directing compoundand solvent. It has been found that particularly useful ratios of thenumber of moles of cluster compound compared to the total number ofmoles of the first and second precursor species preferably lie in therange 0.0001-0.1 (no. moles of cluster compound): 1 (total no. moles offirst and second precursor species), more preferably 0.001-0.1:1, yetmore preferably 0.001-0.060:1. Further preferred ratios of the number ofmoles of cluster compound compared to the total number of moles of thefirst and second precursor species lie in the range 0.002-0.030:1, andmore preferably 0.003-0.020:1. In particular, it is preferred that theratio of the number of moles of cluster compound compared to the totalnumber of moles of the first and second precursor species lies in therange 0.0035-0.0045:1.

It is envisaged that any suitable molar ratio of the first precursorspecies compared to the second precursor species may be used. Forexample, the molar ratio of the first precursor species compared to thesecond precursor species may lie in the range 100-1 (first precursorspecies): 1 (second precursor species), more preferably 50-1:1. Furtherpreferred ranges of the molar ratio of the first precursor speciescompared to the second precursor species lie in the range 40-5:1, morepreferably 30-10:1. In certain applications it is preferred thatapproximately equal molar amounts of the first and second precursorspecies are used in the method of the invention. The molar ratio of thefirst precursor species compared to the second precursor speciespreferably lies in the range 0.1-1.2:1, more preferably, 0.9-1.1:1, andmost preferably 1:1. In other applications, it may be appropriate to useapproximately twice the number of moles of one precursor speciescompared to the other precursor species. Thus the molar ratio of thefirst precursor species compared to the second precursor species may liein the range 0.4-0.6:1, more preferably the molar ratio of the firstprecursor species compared to the second precursor species is 0.5:1. Itis to be understood that the above precursor molar ratios may bereversed such that they relate to the molar ratio of the secondprecursor species compared to the first precursor species. Accordingly,the molar ratio of the second precursor species compared to the firstprecursor species may lie in the range 100-1 (second precursor species):1 (first precursor species), more preferably 50-1:1, 40-5:1, or 30-10:1.Furthermore, the molar ratio of the second precursor species compared tothe first precursor species may lie in the range 0.1-1.2:1, 0.9-1.1:1,0.4-0.6:1, or may be 0.5:1.

In a preferred embodiment of the first aspect of the present inventionthe molecular cluster compound and core precursor composition aredispersed in a suitable dispersing medium at a first temperature and thetemperature of the dispersing medium containing the cluster compound andcore precursor composition is then increased to a second temperaturewhich is sufficient to initiate seeding and growth of the nanoparticlecores on the molecular clusters of said compound.

Preferably the first temperature is in the range 50° C. to 100° C., morepreferably in the range 70° C. to 80° C., and most preferably the firsttemperature is approximately 75° C.

The second temperature may be in the range 120° C. to 280° C. Morepreferably the second temperature is in the range 150° C. to 250° C.,and most preferably the second temperature is approximately 200° C.

The temperature of the dispersing medium containing the cluster compoundand core precursor composition may be increased from the firsttemperature to the second temperature over a time period of up to 48hours, more preferably up to 24 hours, yet more preferably 1 hour to 24hours, and most preferably over a time period in the range 1 hour to 8hours.

In a further preferred embodiment of the first aspect of the presentinvention the method comprises

a. dispersing the molecular cluster compound and an initial portion ofthe nanoparticle core precursor composition which is less than the totalamount of the nanoparticle core precursor composition to be used toproduce said nanoparticle cores in a suitable dispersing medium at afirst temperature;

b. increasing the temperature of the dispersing medium containing thecluster compound and core precursor composition to a second temperaturewhich is sufficient to initiate seeding and growth of the nanoparticlecores on the molecular clusters of said molecular cluster compound; and

c. adding one or more further portions of the nanoparticle coreprecursor composition to the dispersing medium containing the growingnanoparticle cores,

wherein the temperature of the dispersing medium containing the growingnanoparticle cores is increased before, during and/or after the additionof the or each further portion of the nanoparticle core precursorcomposition.

In this preferred embodiment less than the total amount of precursor tobe used to produce the nanoparticle cores is present in the dispersingmedium with the cluster compound prior to the initiation of nanoparticlegrowth and then as the reaction proceeds and the temperature isincreased, additional amounts of core precursors are periodically addedto the reaction mixture in the dispersing medium. Preferably theadditional core precursors are added either dropwise as a solution or asa solid.

The temperature of the dispersing medium containing the growingnanoparticle cores may be increased at any appropriate rate dependingupon the nature of the nanoparticle core precursor composition and themolecular cluster compound being used. Preferably the temperature of thedispersing medium is increased at a rate in the range 0.05° C./min to 1°C./min, more preferably at a rate in the range 0.1° C./min to 1° C./min,and most preferably the temperature of the dispersing medium containingthe growing nanoparticle cores is increased at a rate of approximately0.2° C./min.

While the first and second temperatures of the dispersing medium maytake any suitable value, in a preferred embodiment of the presentinvention said first temperature is in the range 15° C. to 60° C. Saidsecond temperature may be in the range 90° C. to 150° C.

It is preferred that the or each further portion of the nanoparticlecore precursor composition is added dropwise to the dispersing mediumcontaining the growing nanoparticle cores.

The or each further portion of the nanoparticle core precursorcomposition may be added to the dispersing medium containing the growingnanoparticle cores at any desirable rate. It is preferred that the coreprecursor composition is added to the dispersing medium at a rate in therange 0.1 ml/min to 20 ml/min per litre of dispersing medium, morepreferably at a rate in the range 1 ml/min to 15 ml/min per litre ofdispersing medium, and most preferably at a rate of around 5 ml/min perlitre of dispersing medium.

Preferably said initial portion of the nanoparticle core precursorcomposition is less than or equal to approximately 90% of the totalamount of the nanoparticle core precursor composition to be used toproduce said nanoparticle cores. Said initial portion of thenanoparticle core precursor composition may be less than or equal toapproximately 10% of the total amount of the nanoparticle core precursorcomposition to be used to produce said nanoparticle cores.

In a preferred embodiment where one further portion of the nanoparticlecore precursor composition is added to the dispersing medium containingthe growing nanoparticle cores said one further portion is less than orequal to approximately 90% of the total amount of the nanoparticle coreprecursor composition to be used to produce said nanoparticle cores.

In a further preferred embodiment where more than one further portion ofthe nanoparticle core precursor composition is added to the dispersingmedium containing the growing nanoparticle cores, each of said furtherportions is less than or equal to approximately 45% of the total amountof the nanoparticle core precursor composition to be used to producesaid nanoparticle cores. Each of said further portions may be less thanor equal to approximately 10% of the total amount of the nanoparticlecore precursor composition to be used to produce said nanoparticlecores.

It is preferred that formation of said molecular cluster compound iseffected in situ in said dispersing medium prior to dispersing themolecular cluster compound and the initial portion of the nanoparticlecore precursor composition in said dispersing medium.

In a preferred embodiment of the present invention said process issubject to the proviso that the nanoparticle core precursor compositiondoes not contain Cd(CH₃CO₂)₂. A further preferred embodiment providesthat said process is subject to the proviso that the nanoparticle coreprecursor composition does not contain TOPSe. Said process may besubject to the proviso that the nanoparticle core precursor compositiondoes not contain Cd(CH₃CO₂)₂ and TOPSe. In a still further preferredembodiment said process is subject to the proviso that the temperatureof the dispersing medium containing the growing nanoparticle cores isincreased at a rate which is other than 50° C. over a period of 24hours.

The conversion of the core precursor to the material of thenanoparticles can be conducted in any suitable dispersing medium orsolvent. In the method of the present invention it is important tomaintain the integrity of the molecules of the cluster compound.Consequently, when the cluster compound and nanoparticle core precursorare introduced in to the dispersing medium or solvent the temperature ofthe medium/solvent must be sufficiently high to ensure satisfactorydissolution and mixing of the cluster compound it is not necessary thatthe present compounds are fully dissolved but desirable. It is mostpreferred that the temperature of the dispersing medium containing thecluster and precursors should not be so high as to disrupt the integrityof the cluster compound molecules. Once the cluster compound and coreprecursor composition are sufficiently well dissolved in the solvent thetemperature of the solution thus formed is raised to a temperature, orrange of temperatures, which is/are sufficiently high to initiatenanoparticle core growth but not so high as to damage the integrity ofthe cluster compound molecules. As the temperature is increased furtherquantities of core precursor are added to the reaction, preferably in adropwise manner or as a solid. The temperature of the solution can thenbe maintained at this temperature or within this temperature range foras long as required to form nanoparticle cores possessing the desiredproperties.

A wide range of appropriate dispersing media/solvents are available. Theparticular dispersing medium used is usually at least partly dependentupon the nature of the reacting species, i.e. nanoparticle coreprecursor and/or cluster compound, and/or the type of nanoparticleswhich are to be formed. Preferred dispersing media include Lewis basetype coordinating solvents, such as a phosphine (e.g. TOP), a phosphineoxide (e.g. TOPO) or an amine (e.g. HDA), or non-coordinating organicsolvents, e.g. alkanes and alkenes (e.g. octadecene). If anon-coordinating solvent is used then it will usually be used in thepresence of a further coordinating agent to act as a capping agent forthe following reason.

If the nanoparticles being formed are intended to function as quantumdots it is important that the surface atoms which are not fullycoordinated “dangling bonds” are capped to minimise non-radiativeelectron-hole recombinations and inhibit particle agglomeration whichcan lower quantum efficiencies or form aggregates of nanoparticles. Anumber of different coordinating solvents are known which can also actas capping or passivating agents, e.g. TOP, TOPO, HDA or long chainorganic acids such as myristic acid. If a solvent is chosen which cannotact as a capping agent then any desirable capping agent can be added tothe reaction mixture during nanoparticle growth. Such capping agents aretypically Lewis bases but a wide range of other agents are available,such as oleic acid and organic polymers which form protective sheathsaround the nanoparticles.

The first aspect of the present invention comprises of a method toproduce nanoparticle materials using molecular clusters, whereby theclusters are defined identical molecular entities, as compared toensembles of small nanoparticles, which inherently lack the anonymousnature of molecular clusters. The invention consists of the use ofmolecular clusters as templates to seed the growth of nanoparticlecores, whereby other molecular sources, i.e. the precursor compounds, or“molecular feedstocks” are consumed to facilitate particle growth. Themolecular sources (i.e. core precursor composition) are periodicallyadded to the reaction solution so as to keep the concentration of freeions to a minimum but also maintain a concentration of free ions toinhibit Oswald ripening from occurring and defocusing of nanoparticlesize range from occurring.

A further preferred embodiment of the first aspect of the presentinvention provides that the method comprises:

i. monitoring the average size of the nanoparticle cores being grown;and

ii. terminating nanoparticle core growth when the average nanoparticlesize reaches a predetermined value.

It is preferred that the average size of the nanoparticle cores beinggrown is monitored by UV-visible absorption spectroscopy. The averagesize of the nanoparticle cores being grown may be monitored byphotoluminescence spectroscopy. Preferably nanoparticle core growth isterminated by reducing the temperature of the dispersing medium from thesecond temperature to a third temperature.

Conveniently the method may comprise forming a precipitate of thenanoparticle core material by the addition of a precipitating reagent,which is preferably selected from the group consisting of ethanol andacetone.

Preferably conversion of the core precursor composition to thenanoparticle core is effected in a reaction medium and said nanoparticlecore is isolated from said reaction medium prior to deposition of thefirst layer.

It is preferable that deposition of said first layer comprises effectingconversion of a first semiconductor material precursor composition tosaid first semiconductor material. The first semiconductor materialprecursor composition preferably comprises third and fourth precursorspecies containing the ions to be incorporated into the growing firstlayer of the nanoparticle. The third and fourth precursor species may beseparate entities contained in said first semiconductor materialprecursor composition, or the third and fourth precursor species may becombined in a single entity contained in the first semiconductormaterial precursor composition.

Preferably deposition of said second layer comprises effectingconversion of a second semiconductor material precursor composition tosaid second semiconductor material. The second semiconductor materialprecursor composition preferably comprises fifth and sixth precursorspecies containing the ions to be incorporated into the growing secondlayer of the nanoparticle. It is preferred that the fifth and sixthprecursor species are separate entities contained in said secondsemiconductor material precursor composition, alternatively the fifthand sixth precursor species may be combined in a single entity containedin said second semiconductor material precursor composition.

A second aspect of the present invention provides a nanoparticleproduced according to a method in accordance with the first aspect ofthe present invention.

A third aspect of the present invention provides a nanoparticlecomprised of a core comprising a core semiconductor material, a firstlayer comprising a first semiconductor material provided on said coreand a second layer comprising a second semiconductor material providedon said first layer, said core semiconductor material being different tosaid first semiconductor material and said first semiconductor materialbeing different to said second semiconductor material, wherein

a) at least two of the core, first shell and second shell materialsincorporate ions from groups 12 and 15 of the periodic table, groups 14and 16 of the periodic table, or groups 11, 13 and 16 of the periodictable;

b) the second shell material incorporates ions of at least two differentelements from group 12 of the periodic table and ions from group 16 ofthe periodic table;

c) at least one of the core, first and second semiconductor materialsincorporates ions from groups 11, 13 and 16 of the periodic table and atleast one other of the core, first and second semiconductor materials isa semiconductor material not incorporating ions from groups 11, 13 and16 of the periodic table.

Preferably in set a) the other of the core, first and secondsemiconductor materials incorporates ions from the group consistinggroups 12 and 15 of the periodic table, groups 13 and 15 of the periodictable, groups 12 and 16 of the periodic table, groups 14 and 16 of theperiodic table, and groups 11, 13 and 16 of the periodic table.

It is preferred that in set b) said second semiconductor material hasthe formula M_(x)N_(1-x)E, where M and N are the group 12 ions, E is thegroup 16 ion, and 0<x<1. It is preferred that 0.1<x<0.9, more preferably0.2<x<0.8, and most preferably 0.4<x<0.6. Particularly preferrednanoparticles have the structure ZnS/CdSe/Cd_(x)Zn_(1-x)S,Cd_(x)Zn_(1-x)S/CdSe/ZnS or Cd_(x)Zn_(1-x)S/CdSe/Cd_(x)Zn_(1-x)S.

In a preferred embodiment of set c) said at least one other of the core,first and second semiconductor materials not incorporating ions fromgroups 11, 13 and 16 of the periodic table incorporates ions from thegroup consisting of groups 12 and 15 of the periodic table, groups 13and 15 of the periodic table, groups 12 and 16 of the periodic table,and groups 14 and 16 of the periodic table.

Preferably the nanoparticle further comprises a third layer of a thirdsemiconductor material provided on said second layer. The nanoparticlemay optionally comprise still further layers of semiconductor material,such as fourth, fifth, and sixth layers.

Regarding the third aspect of the present invention it is preferred thatthe third semiconductor material is selected from the group consistingof a semiconductor material incorporating ions from groups 12 and 15 ofthe periodic table, a semiconductor material incorporating ions fromgroups 13 and 15 of the periodic table, a semiconductor materialincorporating ions from groups 12 and 16 of the periodic table, asemiconductor material incorporating ions from groups 14 and 16 of theperiodic table and a semiconductor material incorporating ions fromgroups 11, 13 and 16 of the periodic table.

Preferably the group 12 ions are selected from the group consisting ofzinc ions, cadmium ions and mercury ions.

The group 15 ions are preferably selected from the group consisting ofnitride ions, phosphide ions, arsenide ions, and antimonide ions.

It is preferred that the group 14 ions are selected from the groupconsisting of lead ions, tin ions and germanium ions.

Preferably the group 16 ions are selected from the group consisting ofsulfide ions, selenide ions and telluride ions.

The group 11 ions are preferably selected from the group consisting ofcopper ions, silver ions and gold ions.

In a preferred embodiment the group 13 ions are selected from the groupconsisting of aluminium ions, indium ions and gallium ions.

The current invention describes the design and preparation methods of anumber of unique quantum dot—quantum wells nanoparticles including,ZnS/CuInS₂/ZnS, ZnS/CuInS₂/Cd_(x)Zn_(1-x)S,Cd_(x)Zn_(1-x)S/CuInS₂/Cd_(x)Zn_(1-x)S, ZnS/CuGaS₂/ZnS,ZnS/CuGaS₂/Cd_(x)Zn_(1-x)S, Cd_(x)Zn_(1-x)S/CuGaS₂/Cd_(x)Zn_(1-x)S,ZnS/CuInSe₂/ZnS, ZnS/CuInSe₂/Cd_(x)Zn_(1-x)S,Cd_(x)Zn_(1-x)S/CuInSe₂/Cd_(x)Zn_(1-x)S, ZnS/CuGaSe₂/ZnS,ZnS/CuGaSe₂/Cd_(x)Zn_(1-x)S and Cd_(x)Zn_(1-x)S/CuGaSe₂/Cd_(x)Zn_(1-x)S,where 0<x<1.

A fourth aspect of the present invention provides a method for producinga nanoparticle according to the third aspect of the present invention,wherein the method comprises effecting conversion of a nanoparticle coreprecursor composition to the material of the nanoparticle core,depositing said first layer on said core and depositing said secondlayer on said first layer.

It will be evident to the skilled person how the method forming thefourth aspect of the present invention may be put in to effect byroutine modification to the experimental details disclosed herein andinvolving no undue experimentation for the preparation ofcore/multishell nanoparticles in accordance with the third aspect of thepresent invention.

Preferably said nanoparticle core precursor composition comprises firstand second core precursor species containing the ions to be incorporatedinto the growing nanoparticle core. It is preferred that the first andsecond core precursor species are separate entities contained in thecore precursor composition, and the conversion is effected in thepresence of a molecular cluster compound under conditions permittingseeding and growth of the nanoparticle core.

The first and second core precursor species may be combined in a singleentity contained in the core precursor composition.

Preferably conversion of the core precursor composition to thenanoparticle core is effected in a reaction medium and said nanoparticlecore is isolated from said reaction medium prior to deposition of thefirst layer.

In a preferred embodiment of the fourth aspect of the present inventiondeposition of the first layer comprises effecting conversion of a firstsemiconductor material precursor composition to said first semiconductormaterial.

Preferably the first semiconductor material precursor compositioncomprises third and fourth precursor species containing the ions to beincorporated into the growing first layer of the nanoparticle. The thirdand fourth precursor species may be separate entities contained in thefirst semiconductor material precursor composition (i.e. the precursorspecies may be provided by a ‘multi source’ or ‘dual source’ precursorcomposition). Alternatively or additionally the third and fourthprecursor species may be combined in a single entity contained in thefirst semiconductor material precursor composition (i.e. the precursorcomposition may contain a ‘single source’ precursor comprising both thethird and fourth ions to be incorporated in to the first layer).

Preferably deposition of the second layer comprises effecting conversionof a second semiconductor material precursor composition to said secondsemiconductor material.

Preferably the second semiconductor material precursor compositioncomprises fifth and sixth precursor species containing the ions to beincorporated into the growing second layer of the nanoparticle. Thefifth and sixth precursor species may be separate entities contained insaid second semiconductor material precursor composition, and/or thefifth and sixth precursor species may be combined in a single entitycontained in said second semiconductor material precursor composition.

The invention addresses a number of problems, which include thedifficulty of producing high efficiency blue emitting dots.

The most researched and hence best-characterized semiconductor QD isCdSe, whose optical emission can be tuned across the visible region ofthe spectrum. Green and red CdSe/ZnS core-shell nanocrystals are themost widely available under existing methodologies. CdSe nanoparticleswith blue emission along with narrow spectral widths and highluminescence quantum yields are difficult to synthesize using theconventional high temperature rapid injection “nucleation and growth”method. Using this conventional method to make blue quantum dots isdifficult as the blue quantum dots are the smallest and are what isinitially formed but rapidly grow (about 3 seconds of reaction time) into larger does which have a green emission. There are also furtherproblems including difficulties in experimental work-up, processes andovercoating with ZnS. Moreover, only small quantities of material can beproduced in a single batch due to the dilute reaction solution necessaryto keep the particle size small. Alternative blue emitting semiconductornanocrystals include ZnTe and CdS, however, growing large (>4.5 nmdiameter) ZnTe, needed for blue emissions, with narrow sizedistributions has proved difficult.

CdS on the other hand has an appropriate band gap and has been shown toemit in the 460-480 nm range with narrow size distributions and goodluminescence efficiency. Bare CdS cores tend to emit white luminescence,attributed to deep trap emissions which can be suppressed by overcoatingby a wide band gap material such as ZnS. These CdS/ZnS structures haveshown recent promise as the active material for blue QD LED's and blueQD lasers.

Quantum Dots Incorporating Lower Toxicity Elements

Another drive for designing and producing specific quantum dot-quantumwell structures in this invention is the current need for quantum dotsfree of elements (e.g. cadmium and mercury) which are deemed by nationalauthorities to be toxic or potentially toxic but which have similaroptical and/or electronic properties to those of CdSe—ZnS core-shellquantum dots. The current invention includes the design and synthesis ofa number of cadmium free QD-QW structures based onII-VI/I-III-VI₂/II-VI, III-V/II-VIII-V materials such as but notrestricted to ZnS/CuInS₂/ZnS, ZnS/CuGaS₂/ZnS, ZnS/CuInSe₂/ZnS,ZnS/CuGaSe₂/ZnS.

Current Synthetic Methods

Many synthetic methods for the preparation of semiconductornanoparticles have been reported, early routes applied conventionalcolloidal aqueous chemistry, with more recent methods involving thekinetically controlled precipitation of nanocrystallites, usingorganometallic compounds.

Over the past six years the important issues have concerned thesynthesis of high quality semiconductor nanoparticles in terms ofuniform shape, size distribution and quantum efficiencies. This has leadto a number of methods that can routinely produce semiconductornanoparticles, with monodispersity of <5% with quantum yields >50%. Mostof these methods are based on the original “nucleation and growth”method described by Murray, Norris and Bawendi, using organometallicprecursors. Murray et at originally used organometallic solutions ofmetal-alkyls (R₂M) M=Cd, Zn, Te; R=Me, Et and tri-n-octylphosphinesulfide/selenide (TOPS/Se) dissolved in tri-n-octylphosphine (TOP).These precursor solutions are injected into hot tri-n-octylphosphineoxide (TOPO) in the temperature range 120-400° C. depending on the sizeof the particles required and the material being produced. This producesTOPO coated/capped semiconductor nanoparticles of II-VI material. Thesize of the particles is controlled by the temperature, concentration ofprecursor used and length of time at which the synthesis is undertaken,with larger particles being obtained at higher temperatures, higherprecursor concentrations and prolonged reaction times.

This organometallic route has advantages over other synthetic methods,including near monodispersity <5% and high particle cystallinity. Asmentioned, many variations of this method have now appeared in theliterature which routinely give high quality core and core-shellnanoparticles with monodispersity of <5% and quantum yield >50% (forcore-shell particles of as-prepared solutions), with many methodsdisplaying a high degree of size and shape control.

Recently attention has focused on the use of “greener” precursors whichare less exotic and less expensive but not necessary moreenvironmentally friendly. Some of these new precursors include theoxides, CdO; carbonates MCO₃ M=Cd, Zn; acetates M(CH₃CO₂) M=Cd, Zn andacetylacetanates [CH₃COOCH═C(O⁻)CH₃]₂ M=Cd, Zn; amongst other. (The useof the term “greener” precursors in semiconductor particle synthesis hasgenerally taken on the meaning of cheaper, readily available and easierto handle precursor starting materials, than the originally usedorganometallics which are volatile and air and moisture sensitive, anddoes not necessary mean that “greener precursors” are any moreenvironmentally friendly).

Single-source precursors have also proved useful in the synthesis ofsemiconductor nanoparticle materials of II-VI, as well as other compoundsemiconductor nanoparticles.Bis(dialkyldithio-/diseleno-carbamato)cadmium(II)/zinc(II) compounds,M(E₂CNR₂)₂ (M=Zn or Cd, E=S or Se and R=alkyl), have used a similar‘one-pot’ synthetic procedure, which involved dissolving the precursorin tri-n-octylphosphine (TOP) followed by rapid injection into hottri-n-octylphosphine oxide/tri-n-octylphosphine (TOPO/TOP) above 200° C.Single-source precursors have also been used to produce I-III-VI₂materials i.e. CuInS₂ using (PPH₃)₂CuIn(SEt)₄ dissolved in a mixture ofhexanethiol and dioctylphalate at 200° C. to give hexanethiol coatedCuInS₂.

I-III-VI₂ nanoparticles have also been prepared from multi-sourceprecursors such as in the case of CuInSe₂ prepared from CuCl dissolvedin triethylene and elemental indium and selenium. CuInTe₂ was produce bya similar approach but from using elemental tellurium.

For all the above methods, rapid particle nucleation followed by slowparticle growth is essential for a narrow particle size distribution.All these synthetic methods are based on the original organometallic“nucleation and growth” method by Murray et al, which involves the rapidinjection of the precursors into a hot solution of a Lewis basecoordinating solvent (capping agent) which may also contain one of theprecursors. The addition of the cooler solution subsequently lowers thereaction temperature and assist particle growth but inhibits furthernucleation. The temperature is then maintained for a period of time,with the size of the resulting particles depending on reaction time,temperature and ratio of capping agent to precursor used. The resultingsolution is cooled followed by the addition of an excess of a polarsolvent (methanol or ethanol or sometimes acetone) to produce aprecipitate of the particles that can be isolated by filtration orcentrifugation.

Preparation from single-source molecular clusters, Cooney and co-workersused the cluster [S₄Cd₁₀(SPh)₁₆] [Me₃NH]₄ to produce nanoparticles ofCdS via the oxidation of surface-capping SPh⁻ ligands by iodine. Thisroute followed the fragmentation of the majority of clusters into ionswhich were consumed by the remaining

Another method whereby it is possible to produce large volumes ofquantum dots, eliminated the need for a high temperature nucleationstep. Moreover, conversion of the precursor composition to thenanoparticles is affected in the presence of a molecular clustercompound. Each identical molecule of a cluster compound acts as a seedor nucleation point upon which nanoparticle growth can be initiated. Inthis way, nanoparticle nucleation is not necessary to initiatenanoparticle growth because suitable nucleation sites are alreadyprovided in the system by the molecular clusters. The molecules of thecluster compound act as a template to direct nanoparticle growth. Byproviding nucleation sites which are so much more well defined than thenucleation sites employed in previous work the nanoparticles formed inthis way possess a significantly more well defined final structure thanthose obtained using previous methods. A significant advantage of thismethod is that it can be more easily scaled-up for use in industry thanconventional methods.

The particular solvent used is usually at least partly dependent uponthe nature of the reacting species, i.e. nanoparticle precursor and/orcluster compound, and/or the type of nanoparticles which are to beformed. Typical solvents include Lewis base type coordinating solvents,such as a phosphine (e.g. TOP), a phosphine oxide (e.g. TOPO) or anamine (e.g. HDA), hexanethiol, or non-coordinating organic solvents,e.g. alkanes and alkenes. If a non-coordinating solvent is used then itwill usually be used in the presence of a further coordinating agent toact as a capping agent for the following reason.

If the nanoparticles are intended to function as quantum dots an outercapping agent (e.g. an organic layer) must be attached to stop particleagglomeration from occurring. A number of different coordinatingsolvents are known which can also act as capping or passivating agents,e.g. TOP, TOPO, alkylthiols or HDA. If a solvent is chosen which cannotact as a capping agent then any desirable capping agent can be added tothe reaction mixture during nanoparticle growth. Such capping agents aretypically Lewis bases but a wide range of other agents are available,such as oleic acid and organic polymers which form protective sheathsaround the nanoparticles.

DESCRIPTION OF INVENTION Type of System Covered by the Current Invention

The present invention is directed to the preparation of a number ofsemiconductor nanoparticles which may be considered as falling withinthe class of materials known as quantum dot-quantum wells and includesmaterials within the size range 2-100 nm. The present inventiondescribes the architecture and the preparation of a number ofnanoparticles materials and includes a number of compound semiconductorparticles otherwise referred to as quantum dots-quantum well, includematerial comprising of ZnS/CuInS₂/ZnS, ZnS/CuInS₂/Cd_(x)Zn_(1-x)S,Cd:ZnS/CuInS₂/Cd_(x)Zn_(1-x)S, ZnS/CuGaS₂/ZnS,ZnS/CuGaS₂/Cd_(x)Zn_(1-x)S, Cd_(x)Zn_(1-x)S/CuGaS₂/Cd_(x)Zn_(1-x)S,ZnS/CuInSe₂/ZnS, ZnS/CuInSe₂/Cd_(x)Zn_(1-x)S,Cd_(x)Zn_(1-x)S/CuInSe₂/Cd_(x)Zn_(1-x)S, ZnS/CuGaSe₂/ZnS,ZnS/CuGaSe₂/Cd_(x)Zn_(1-x)S and Cd_(x)Zn_(1-x)S/CuGaSe₂/Cd_(x)Zn_(1-x)S,where 0<x<1.

II-VI/II-VI/II-VI Material

Comprising a core of a first element from group 12 of the periodic tableand a second element from group 16 of the periodic table, a first layerof material comprising a shell of a first element from group 12 of theperiodic table and a second element from group 16 of the periodic tableand a second layer material comprising a shell of a first element fromgroup 12 of the periodic table and a second element from group 16 of theperiodic table and also including ternary and quaternary materials anddoped materials. Nanoparticle materials include but are not restrictedto:—ZnS/CdSe/CdS/ZnS, ZnS/CdTe/ZnS, ZnS/CdHgS/ZnS, ZnS/HgSe/ZnS,ZnS/HgTe/ZnS, ZnSe/CdSe/ZnSe, ZnSe/CdTe/ZnSe, ZnSe/HgS/ZnSe,ZnS/HgSe/ZnS, ZnSe/HgTe/ZnSe, ZnTe/CdSe/ZnS, ZnTe/CdTe/ZnS,ZnTe/CdHgS/ZnS, ZnTe/HgSe/ZnS, ZnTe/HgTe/ZnS, CdS/CdSe/ZnS,CdS/CdTe/ZnS, CdS/CdHgS/ZnS, CdS/HgSe/ZnS, CdS/HgTe/ZnS, CdSe/CdTe/ZnS,CdSe/CdHgS/ZnS, CdSe/HgSe/ZnS, CdSe/HgTe/ZnS, CdTe/CdSe/ZnS,CdTe/CdHgS/ZnS, CdTe/HgSe/ZnS, CdTe/HgTe/ZnS, HgS/CdSe/ZnS,HgS/CdTe/ZnS, HgS/CdHgS/ZnS, HgS/HgSe/ZnS, HgS/HgTe/ZnS, HgSe/CdSe/ZnS,HgSe/CdTe/ZnS, HgSe/CdHgS/ZnS, HgSe/HgTe/ZnS.

II-VI/I-III-VI₂II-VI Material

Comprising a core of a first element from group 12 of the periodic tableand a second element from group 16 of the periodic table, a first layerof material comprising of a shell of a first element from group 11 ofthe periodic table and a second element from group 13 of the periodictable a third element from group 16 of the periodic table and a secondlayer material comprising a shell of a first element from group 12 ofthe periodic table and a second element from group 16 of the periodictable and also including ternary and quaternary materials and dopedmaterials. Nanoparticle materials include but are not restricted to:ZnS/CuInS₂/ZnS, ZnS/CuInS₂/CdS/ZnS, CdS/ZnS/CuInS₂/CdS/ZnS,ZnS/CuGaS₂/ZnS, ZnS/CuGaS₂/CdS/ZnS, CdS/ZnS/CuGaS₂/CdS/ZnS,ZnS/CuInSe₂/ZnS, ZnS/CuInSe₂/CdS/ZnS, CdS/ZnS/CuInSe₂/CdS/ZnS,ZnS/CuGaSe₂/ZnS, ZnS/CuGaSe₂/CdS/ZnS, CdS/ZnS/CuGaSe₂/CdS/ZnS.

II-V/II-V/II-V Material

Comprising a core first element from group 12 of the periodic table anda second element from group 15 of the periodic table, a first layercomprising a first element from group 12 of the periodic table and asecond element from group 15 of the periodic table and a second layer ofsemiconductor material comprising a first element from group 12 of theperiodic table and a second element from group 15 of the periodic tableand also including ternary and quaternary materials and doped materials.Nanoparticle material includes but is not restrictedto:—Zn₃P₂/Zn₃As₂/Zn₃P₂, Zn₃P₂/Cd₃P₂/Zn₃P₂, Zn₃P₂/Cd₃As₂/Zn₃P₂,Zn₃P₂/Cd₃N₂/Zn₃P₂, Zn₃P₂/Zn₃N₂/Zn₃P₂, Zn₃As₂/Zn₃P₂/Zn₃As₂,Zn₃As₂/Cd₃P₂/Zn₃As₂, Zn₃As₂/Cd₃As₂/Zn₃As₂, Zn₃As₂/Cd₃N₂/Zn₃As₂,Zn₃As₂/Zn₃N₂/Zn₃As₂, Cd₃P₂/Zn₃P₂/Cd₃P₂, Cd₃P₂/Zn₃As₂/Cd₃P₂,Cd₃P₂/Cd₃As₂/Cd₃P₂, Cd₃P₂/Cd₃N₂/Cd₃P₂, Cd₃P₂/Zn₃N₂/Cd₃P₂,Cd₃As₂/Zn₃P₂/Cd₃As₂, Cd₃As₂/Zn₃As₂/Cd₃As₂, Cd₃As₂/Cd₃P₂/Cd₃As₂,Cd₃As₂/Cd₃N₂/Cd₃As₂, Cd₃As₂/Zn₃N₂/Cd₃As₂, Cd₃N₂/Zn₃P₂/Cd₃N₂,Cd₃N₂/Zn₃As₂/Cd₃N₂, Cd₃N₂/Cd₃P₂/Cd₃N₂, Cd₃N₂/Cd₃As₂/Cd₃N₂,Cd₃N₂/Zn₃N₂/Cd₃N₂, Zn₃N₂/Zn₃P₂/Zn₃N₂, Zn₃N₂/Zn₃As₂/Zn₃N₂,Zn₃N₂/Cd₃P₂/Zn₃N₂, Zn₃N₂/Cd₃As₂/Zn₃N₂, Zn₃N₂/Cd₃N₂/Zn₃N₂.

III-V/III-V/III-V Material

Comprising a core of a first element from group 13 of the periodic tableand a second element from group 15 of the periodic table, a first layercomprising of a first element from group 13 of the periodic table and asecond element from group 15 of the periodic table and a second layercomprising of a first element from group 13 of the periodic table and asecond element from group 15 of the periodic table and also includingternary and quaternary materials and doped materials. Nanoparticlematerials include but are not restricted to:—AlP/AlAs/AlP, AlP/AlSb/AlP,AlP/GaN/AlP, AlP/GaP/AlP, AlP/GaAs/AlP, AlP/GaSb/AlP, AlP/InN/AlP,AlP/InP/AlP, AlP/InAs/AlP, AlP/InSb/AlP, AlAs/AlP/AlAs, AlP/AlSb/AlP,AlP/GaN/AlP, AlP/GaP/AlP, AlP/GaAs/AlP, AlP/GaSb/AlP, AlP/InN/AlP,AlP/InP/AlP, AlP/InAs/AlP, AlP/InSb/AlP, AlSb/AlP/AlSb, AlSb/AlAs/AlSb,AlSb/GaN/AlSb, AlSb/GaP/AlSb, AlSb/GaAs/AlSb, AlSb/GaSb/AlSb,AlSb/InN/AlSb, AlSb/InP/AlSb, AlSb/InAs/AlSb, AlSb/InSb/AlSb,GaN/AlP/GaN, GaN/AlAs/GaN, GaN/AlAs/GaN, GaN/GaP/GaN, GaN/GaAs/GaN,GaN/GaSb/GaN, GaN/InN/GaN, GaN/InP/GaN, GaN/InAs/GaN, GaN/InSb/GaN,GaP/AlP/GaP, GaP/AlAs/GaP, GaP/AlSb/GaP, GaP/GaN/GaP, GaP/GaAs/GaP,GaP/GaSb/GaP, GaP/InNGaP, GaP/InP/GaP, GaP/InAs/GaP, GaP/InSb/GaP,GaAs/AlP/GaAs, GaAs/AlAs/GaAs, GaAs/AlSb/GaAs, GaAs/GaN/GaAs,GaAs/GaP/GaAs, GaAs/GaSb/GaAs, GaAs/InN/GaAs, GaAs/InP/GaAs,GaAs/InAs/GaAs, GaAs/InSb/GaAs, GaSb/AlP/GaSb, GaSb/AlAs/GaSb,GaSb/AlSb/GaSb, GaSb/GaN/GaSb, GaSb/GaP/GaSb, GaSb/GaAs/GaSb,GaSb/InN/GaSb, GaSb/InP/GaSb, GaSb/InAs/GaSb, GaSb/InSb/GaSb,InN/AlP/InN, InN/AlAs/InN, InN/AlSb/InN, InN/GaN/InN, InN/GaP/InN,InN/GaAs/InN, InN/GaSb/InN, InN/InP/InN, InN/InAs/InN, InN/InSb/InN,InP/AlP/InP, InP/AlAs/InP, InP/AlSb/InP, InP/GaN/InP, InP/GaP/InP,InP/GaAs/InP, InP/GaSb/InP, InP/InN/InP, InP/InAs/InP, InP/InSb/InP,InAs/AlP/InAs, InAs/AlAs/InAs, InAs/AlSb/InAs, InAs/GaN/InAs,InAs/GaP/InAs, InAs/GaAs/InAs, InAs/GaSb/InAs, InAs/InN/InAs,InAs/InP/InAs, InAs/InSb/InAs, InSb/AlP/InSb, InSb/AlAs/InSb,InSb/AlSb/InSb, InSb/GaN/InSb, InSb/GaP/InSb, InSb/GaAs/InSb,InSb/GaSb/InSb, InSb/InN/InSb, InSb/InP/InSb, InSb/InAs/InSb.

IV-VI/IV-VI/IV-VI Material

Comprising a core semiconductor material comprising of a first elementfrom group 14 of the periodic table and a second element from group 16of the periodic table, a first layer comprising of a first element fromgroup 14 of the periodic table and a second element from group 16 of theperiodic table and a second layer comprising of a first element fromgroup 14 of the periodic table and a second element from group 16 of theperiodic table and also including ternary and quaternary materials anddoped materials. Nanoparticle materials include but are not restrictedto:—PbS/PbSe/PbS, PbS/PbTe/PbS, PbS/Sb₂Te₃/PbS, PbS/SnS/PbS,PbS/SnSe/PbS, PbS/SnTe/PbS, PbSe/PbS/PbSe, PbSe/PbTe/PbSe,PbSe/Sb₂Te₃/PbSe, PbSe/SnS/PbSe, PbSe/SnSe/PbSe, PbSe/SnTe/PbSe,PbTe/PbS/PbTe, PbTe/PbSe/PbTe, PbTe/Sb₂Te₃/PbTe, PbTe/SnS/PbTe,PbTe/SnSe/PbTe, PbTe/SnTe/PbTe, Sb₂Te₃/PbS/Sb₂Te₃, Sb₂Te₃/PbSe/Sb₂Te₃,Sb₂Te₃/PbTe/Sb₂Te₃, Sb₂Te₃/SnS/Sb₂Te₃, Sb₂Te₃/SnSe/Sb₂Te₃,Sb₂Te₃/SnTe/Sb₂Te₃, SnS/PbS/SnS, SnS/PbSe/SnS, SnS/PbTe/SnS,SnS/Sb₂Te₃/SnS, SnS/SnSe/SnS, SnS/SnTe/SnS, SnSe/PbSe/SnSe,SnSe/PbS/SnSe, SnSe/PbTe/SnSe, SnSe/Sb₂Te₃/SnSe, SnSe/SnS/SnSe,SnSe/SnTe/SnSe, SnTe/PbS/SnTe, SnTe/PbSe/SnTe, SnTe/PbTe/SnTe,SnTe/Sb₂Te₃/SnTe, SnTe/SnS/SnTe, SnTe/SnSe/SnTe.

DEFINITIONS RELATING TO THE INVENTION Semiconductor Nanoparticle

Semiconductor nanoparticles are also known as nanocrystals or quantumdots and generally possess a core surrounded by at least one shell ofsemiconductor material. Nanoparticles comprising a core and a pluralityof shells are known as core/multi-shell nanoparticles. An importantclass of core/multi-shell nanoparticles are quantum dot-quantum wellswhich possess an architecture whereby there is a central core of onematerial overlaid by another material which is further over layered byanother material in which adjacent layers comprise differentsemiconductor materials.

Ternary Phase

By the term ternary phase nanoparticle for the purposes ofspecifications and claims, refer to nanoparticles of the above buthaving a core and/or at least one shell layer comprising a threecomponent material. The three components are usually compositions ofelements from the as mentioned groups, for example(Zn_(x)Cd_((1-x)m)L_(n) nanocrystal (where L is a capping agent and0<x<1).

Quaternary Phase

By the term quaternary phase nanoparticle for the purposes ofspecifications and claims, refer to nanoparticles of the above buthaving a core or at least one shell comprising a four-componentmaterial. The four components are usually compositions of elements fromthe as mentioned groups, example being (Zn_(x)Cd_(x-1)S_(y)Se_(y-1))Lnanocrystal (where L is a capping agent, 0<x<1 and 0<y<1).

Solvothermal

By the term Solvothermal for the purposes of specifications and claims,refer to heating the reaction solution so as to initiate and sustainparticle growth or to initiate a chemical reaction between precursors toinitiate particle growth and can also take the meaning solvothermal,thermolysis, thermolsolvol, solution-pyrolysis, lyothermal.

Core-Shell and Core/Multi Shell(Quantum Dot-Quantum Well) Particles

The material used on any shell or subsequent numbers of shells grownonto the core particle in most cases will be of a similar lattice typematerial to the core material i.e. have close lattice match to the corematerial so that it can be epitaxially grown on to the core, but is notnecessarily restricted to materials of this compatibility. The materialused on any shell or subsequent numbers of shells grown on to the corepresent in most cases will have a wider band-gap then the core materialbut is not necessarily restricted to materials of this compatibility.

Capping Agent

The outer most layer (capping agent) of organic material or sheathmaterial is to inhibit particles aggregation and to protect thenanoparticle from the surrounding chemical environment and to provide ameans of chemical linkage to other inorganic, organic or biologicalmaterial. The capping agent can be the solvent that the nanoparticlepreparation is undertaken in, and consists of a Lewis base compoundwhereby there is a lone pair of electrons that are capable of donor typecoordination to the surface of the nanoparticle and can include mono- ormulti-dentate ligands of the type but not restricted to:—phosphines(trioctylphosphine, triphenolphosphine, t-butylphosphine), phosphineoxides (trioctylphosphine oxide), alkyl-amine (hexadecylamine,octylamine), ary-amines, pyridines, and thiophenes.

The outer most layer (capping agent) can consist of a coordinated ligandthat processes a functional group that can be used as a chemical linkageto other inorganic, organic or biological material such as but notrestricted to:—mercaptofunctionalized amines or mercaptocarboxylicacids.

The outer most layer (capping agent) can consist of a coordinated ligandthat processes a functional group that is polymerisable and can be usedto form a polymer around the particle, polymerisable ligands such as butnot limited to styrene functionalized amine, phosphine or phosphineoxide ligand.

Nanoparticle Shape

The shape of the nanoparticle is not restricted to a sphere and canconsist of but not restricted to a rod, sphere, disk, tetrapod or star.The control of the shape of the nanoparticle is by the addition of acompound that will preferentially bind to a specific lattice plane ofthe growing particle and subsequently inhibit or slow particle growth ina specific direction. Example of compounds that can be added but is notrestricted to include:—phosphonic acids (n-tetradecylphosphonic acid,hexylphosphonic acid, 1-decanesulfonic acid, 12-hydroxydodecanoic acid,n-octadecylphosphonic acid).

Description of Preparative Procedure

The current invention should lead to pure, monodispersed,nanocrystalline particles of the materials as described above, that arestabilized from particle aggregation and the surrounding chemicalenvironment by a capping agent, such as an organic layer.

Synthetic Method Employed

The synthetic method employed to produce the initial core and core-shellmaterial can either be by the conventional method of high temperaturerapid injection “nucleation and growth” as in the fourth aspect of thepresent invention or where larger quantities of material is required bya seeding process using of a molecular cluster with dual precursors inaccordance with the first and fourth aspects of the present invention.

Further consecutive treatment of the as formed nanoparticles (ZnS andCd_(x)Zn_(1-x)S) to form core-shell and then quantum dot-quantum wellparticles may be undertaken. Core-shell particle preparation isundertaken either before or after nanoparticle isolation, whereby thenanoparticles are isolated from the reaction and redissolved in new(clean) capping agent/solvent, this can result in a better quantumyield.

For II-VI material, a source for II and a source for VI precursor areadded to the reaction mixture and can be either in the form of twoseparate precursors one containing I element, and the other containingVI element or as a single-source precursor that contains both II and VIwithin a single molecule to form a core or shell layer of II-VI material(e.g. where II=Cd, Zn, VI=S, Se).

For I-III-VI₂ material, a source for I (group 11 of the periodic table),a source for III and a source for VI element precursor are added to thereaction mixture and can be either in the form of three separateprecursors one containing I element, one containing III element and theother containing VI or as a single-source precursor that contains both Iand VI and III and VI within a single molecules to form the I-III-VI₂layer (where I═Cu and III=In, Ga and VI=S, Se), or a single-sourceprecursor which contains all three elements.

For II-V material, a source for II and a source for V precursor areadded to the reaction mixture and can be either in the form of twoseparate precursors one containing a group II element, and the othercontaining V element or as a single-source precursor that contains bothII and V within a single molecule to form a core or shell layer of II-Vmaterial (where II=Zn, Cd, Hg V═N, P, As, Sb, Bi).

For III-V material, a source for III and a source for V precursor areadded to the reaction mixture and can be either in the form of twoseparate precursors one containing III, and the other containing V or asa single-source precursor that contains both III and V within a singlemolecules to form a core or shell layer of III-V material (where III=In,Ga, Al, B, V═N, P, As, Sb, Bi).

For IV-VI material, a source for IV and a source for VI precursor areadded to the reaction mixture and can be either in the form of twoseparate precursors one containing IV element, and the other containingVI element or as a single-source precursor that contains both IV and VIwithin a single molecule to form a core or shell layer of IV-VI material(where IV=Si, C, Ge, Sn, Pb VI=S, Se, Te).

The process may be repeated with the appropriate element precursorsuntil the desired quantum dot-quantum well or core/multi-shell materialis formed. The nanoparticles size and size distribution in an ensembleof particles is dependent on the growth time, temperature andconcentrations of reactants in solution, with higher temperaturesgenerally producing larger nanoparticles.

Precursor Materials Used to Grow the Quantum Dot-Quantum Well StructuresCore Material Source—Multi-Source Precursor Materials Metal Ions

For a compound semiconductor nanoparticle comprising a coresemiconductor material of, for example, (ZnS)L or (Cd_(x)Zn_(1-x)S)L(where L is a ligand or capping agent) a source for element Zn and Cd isfurther added to the reaction and can consist of any Zn or Cd-containingcompound that has the ability to provide the growing particles with asource of Zn or Cd ions. The precursor can comprise but is notrestricted to an organometallic compound, an inorganic salt, acoordination compound or the element.

Examples for II-VI, for the first element include but are not restrictedto:—

Organometallic such as but not restricted to a MR₂ where M=Mg R=alky oraryl group (Mg^(t)Bu₂); MR₂ where M=Zn, Cd; R=alky or aryl group (Me₂Zn,Et₂Zn Me₂Cd, Et₂Cd); MR₃.

Coordination compound such as a carbonate or a β-diketonate orderivative thereof, such as acetylacetonate (2,4-pentanedionate)[CH₃COOCH═C(O—)CH₃]₂ M=Zn, Cd;

Inorganic salt such as but not restricted to a Oxides ZnO, CdO, NitratesMg(NO₃)₂, Cd(NO₃)₂, Zn(NO₃)₂, M(CO₃)₂ M=Zn, Cd; M(CH₃CO₂)₂ M=Zn, Cd,

An element Zn, Cd,

Non-Metal Ions

For a compound semiconductor nanoparticle comprising, for example,(ZnE)_(n)L_(m) or (Cd_(x)Zn_((1-x))E)_(n)L_(m), a source of E ions,where E is a non-metal, for example, sulfur or selenium, is furtheradded to the reaction and can consist of any E-containing compound thathas the ability to provide the growing particles with a source of Eions. n and m are numerical values selected to provide the desiredcompound. L is a ligand, such as a capping agent. The precursor cancomprise but is not restricted to an organometallic compound, aninorganic salt, a coordination compound or an elemental source.

Examples for an II-VI, semiconductor where the second elements includebut are not restricted to:—

ER₂ (E=S or Se; R=Me, Et, ^(t)Bu, ^(i)Bu etc.); HER (E=S or Se; R=Me,Et, ^(t)Bu, ^(i)Bu, ^(i)Pr, Ph etc); thiourea S═C(NH₂)₂.

An element S or Se. An elemental source can be used whereby the elementis directly added to the reaction or is coordinated to a σ-donor Lewisbase compound (two electron pair donor); such as elemental sulfur orselenium coordinating to TOP (tri-octyl-phosphine) to form TOPS andTOPSe respectively or the use of other Lewis bases such as phosphines,amines or phosphine oxides but not restricted to, such as in the case ofusing octylamine to coordinate sulfur.

Core Material Source—Single-Source Precursor Materials

For a compound semiconductor nanoparticle comprising, for example,elements ZnS or Cd_(x)Zn_((1-x))S a source for Zn or Cd and S can be inthe from of a single-source precursor, whereby the precursor to be usedcontains both Zn or Cd and S within a single molecule. This precursorcan be an organometallic compound and inorganic salt or a coordinationcompound, (Zn_(a)S_(b))L_(c) or (Cd_(x)Zn_((1-x))S)_(n)L_(m) Where Zn orCd and S are the elements required within the nanoparticles and L is thecapping ligands.

Examples for an II-VI semiconductor where M=II and E=VI element can bebut is not restricted to bis(dialkyldithio-carbamato)M, (II) complexescompounds of the formula M(S₂CNR₂)₂ M=Zn, Cd; S═S, and R=alkyl or arylgroups; CdS Cd[SSiMe₃]₂, Cd(SCNHNH₂)₂Cl₂, Cd(SOCR)₂.py; [RME^(t)Bu]₅M=Zn, Cd; E=S; R=Me, Et, Ph; [X]₄[E₄M₁₀(SR)₁₆] E=S, M=Zn, Cd; X=Me₃NH⁺,Li⁺, Et₃NH⁺R=Me, Et, Ph; [Cd₃₂S₁₄(SPh)₃₆].L; [M₄(SPh)₁₂]⁺ [X]₂ ⁻M=Zn,Cd, X=Me₄N⁺, Li⁺; [Zn(SEt)Et]₁₀:

[MeMe^(i)Pr] M=Zn, Cd, E=S; [RCdSR′]₅ R═O(ClO₃), R′═PPh₃, ^(i)Pr;[Cd₁₀S₄(S′Ph)₁₂(PR₃)₄]. [(^(t)Bu)GaSe]₄; [^(t)BuGaS]₇; [RInSe]₄R=^(t)Bu, CMe₂Et, Si(^(t)Bu)₃, C(SiMe₃)₃; [RInS]₄ R=^(t)Bu, CMe₂Et;[RGaS]₄ R=^(t)Bu, CMe₂Et, CEt₃; [SAlR′]₄ R═C(SMe₃)₃, CEtMe₂;[(C(SiMe₃)₃)GaS]₄; [^(t)BuGaS]₆; [RGaSe]₄ R=^(t)Bu, CMe₂Et, CEt₃,C(SiMe₃)₃, Cp*, [Cu₁₂Se₆(PR₃)₈] R=Et₂Ph, ^(n)Pr₃, Cy₃.

First Semiconductor Materials For Use in First Layer

For a compound semiconductor quantum dot-quantum well nanoparticlecomprising a first layer of, for example, I-III-VI₂ or II-VI material,sources for element I, III, VI or II are added to the reaction and canconsist of any I, III, VI or II-containing compound that has the abilityto provide the growing particles with a source of E ions. The precursorcan consist of but are not restricted to an organometallic compound, aninorganic salt, a coordination compound or an elemental source. Examplesinclude but are not restricted to:—

Group I Source (e.g. Cu)

But is not restricted to:—CuX where X═Cl, Br, I; Copper(II) acetate(CH₃CO₂)₂Cu, Copper(I) acetate CH₃CO₂Cu, copper(II) acetylacetonate[CH₃COCH═C(O⁻)CH₃]₂Cu and other β-diketonate, copper(I) butanethioateCH₃(CH₂)₃SCu, Copper(II) nitrate Cu(NO₃)₂, CuO.

Group II Source (e.g. Mg)

Organometallic such as but not restricted to a MR₂ where M=Mg R=alky oraryl group (Mg^(t)Bu₂); MR₂ where M=Zn, Cd; R=alky or aryl group (Me₂Zn,Et₂Zn Me₂Cd, Et₂Cd); MR₃.

Coordination compound such as a carbonate or a β-diketonate orderivative thereof, such as acetylacetonate (2,4-pentanedionate)[CH₃COOCH═C(O⁻)CH₃]₂ M=Zn, Cd;

Inorganic salt such as but not restricted to an Oxide, e.g ZnO, CdO, aNitrate, e.g. Mg(NO₃)₂, Cd(NO₃)₂, Zn(NO₃)₂, M(CO₃)₂ M=Zn, Cd; M(CH₃CO₂)₂M=Zn, Cd,

An element Zn, Cd,

Group III Source (e.g. In and Ga)

But is not restricted to:—

MR₃ Where M=Ga, In, Al, B; R=alky or aryl group [AlR₃, GaR₃, InR₃ (R=Me,Et, ^(i)Pr)].

Coordination compound such as a β-diketonate or derivative thereof, suchas [CH₃COOCH═C(O⁻)CH₃]₂ M=Al, Ga, In.

Inorganic salt such as but not restricted to an Oxide, e.g. In₂O₃,Ga₂O₃; a Nitrate, e.g. In(NO₃)₃, Ga(NO₃)₃; M(CH₃C)₃ M=Al, Ga, In

An element Ga, In.

Group VI Source (S or Se)

MR₂ (M=S, Se; R=Me, Et, ^(t)Bu, ^(i)Bu etc.); HMR (M=S, Se; R=Me, Et,^(t)Bu, ^(i)Bu, ^(i)Pr, Ph etc); thiourea S═C(NH₂)₂; Se═C(NH₂)₂.

An element S, Se. An elemental source can be used whereby the element isdirectly added to the reaction or is coordinated to a σ-donor Lewis basecompound (two electron pair donor); such as elemental sulfur or seleniumcoordinating to TOP (tri-octyl-phosphine) to form TOPS and TOPSerespectively or the use of other Lewis bases such as phosphines, aminesor phosphine oxides but not restricted to, such as in the case of usingoctylamine to coordinate sulfur.

First Semiconductor Materials—Single-Source Precursors

Examples for an II-VI semiconductor where M=II and E=VI element can bebut is not restricted to bis(dialkyldithio-carbamato)M, (II) complexescompounds of the formula M(S₂CNR₂)₂ M=Zn, Cd; S═S, and R=alkyl or arylgroups; CdS Cd[SSiMe₃]₂, Cd(SCNHNH₂)₂Cl₂, Cd(SOCR)₂.py; [RME^(t)Bu]₅M=Zn, Cd; E=S₅; R=Me, Et, Ph; [X]₄[E₄M₁₀(SR)₁₆] E=S, M=Zn, Cd; X=Me₃NH⁺,Li⁺, Et₃NH⁺R=Me, Et, Ph; [Cd₃₂S₁₄(SPh)₃₆].L; [M₄(SPh)₁₂]⁺[X]₂ ⁻M=Zn, Cd,X=Me₄N⁺, Li⁺; [Zn(SEt)Et]₁₀:

[MeMe^(i)Pr] M=Zn, Cd, E=S; [RCdSR′], R═O(ClO₃), R′═PPh₃, ^(i)Pr;[Cd₁₀S₄(S′Ph)₁₂(PR₃)₄]. [(^(t)Bu)GaSe]₄; [^(t)BuGaS]₇; [RInSe]₄R=^(t)Bu, CMe₂Et, Si(^(t)Bu)₃, C(SiMe₃)₃; [RInS]₄ R=^(t)Bu, CMe₂Et;[RGaS]₄ R=^(t)Bu, CMe₂Et, CEt₃; [SAlR′]₄ R═C(SMe₃)₃, CEtMe₂;[(C(SiMe₃)₃)GaS]₄; [^(t)BuGaS]₆; [RGaSe]₄ R=^(t)Bu, CMe₂Et, CEt₃,C(SiMe₃)₃, Cp*, [Cu₁₂Se₆(PR₃)₈] R=Et₂Ph, ^(n)Pr₃, Cy₃.

Second Semiconductor Materials For Use in Second, Outer or any OtherSubsequent Layers

The precursor(s) used to provide the second semiconductor material maybe chosen from the same lists of materials set out above in respect ofthe first semiconductor material.

For a quantum dot-quantum well with the second or outer most layercomprising, for example, (ZnS)_(n)L_(m) or (Cd_(x)Zn_((1-x))S)_(n)L_(m)a source for element Zn and Cd is further added to the reaction and canconsist of any Zn or Cd-containing compound that has the ability toprovide the growing particles with a source of Zn or Cd ions. Theprecursor can consist of but are not restricted to an organometalliccompound, an inorganic salt, a coordination compound or the element.

Examples for II-VI, for the first element include but are not restrictedto:—

Organometallic such as but not restricted to a MR₂ where M=Mg R=alky oraryl group (Mg^(t)Bu₂); MR₂ where M=Zn, Cd; R=alky or aryl group (Me₂Zn,Et₂Zn Me₂Cd, Et₂Cd); MR₃.

Coordination compound such as a carbonate or a β-diketonate orderivative thereof, such as acetylacetonate (2,4-pentanedionate)[CH₃COOCH═C(O⁻)CH₃]₂ M=Zn, Cd;

Inorganic salt such as but not restricted to a Oxides ZnO, CdO, NitratesMg(NO₃)₂, Cd(NO₃)₂, Zn(NO₃)₂, M(CO₃)₂ M=Zn, Cd; M(CH₃CO₂)₂ M=Zn, Cd,

An element Zn, Cd.

Non-Metal Ions

For a compound semiconductor nanoparticle comprising, for example(ZnS)_(n)L_(m) or (Cd:ZnS)_(n)L_(m) a source for non-metal ions, E, e.g.sulfur is further added to the reaction and can consist of anyE-containing compound that has the ability to provide the growingparticles with a source of E ions. The precursor can consist of but arenot restricted to an organometallic compound, an inorganic salt, acoordination compound or an elemental source. Examples for an II-VI,semiconductor where the second elements include but are not restrictedto:—

MR₂ (M=S; R=Me, Et, ^(t)Bu, ^(i)Bu etc.); HMR (M=S; R=Me, Et, ^(t)Bu,^(i)Bu, ^(i)Pr, Ph etc); thiourea S═C(NH₂)₂.

An element S or Se. An elemental source can be used whereby the elementis directly added to the reaction or is coordinated to a σ-donor Lewisbase compound (two electron pair donor); such as elemental sulfur orselenium coordinating to TOP (tri-octyl-phosphine) to form TOPS andTOPSe respectively or the use of other Lewis bases such as phosphines,amines or phosphine oxides but not restricted to, such as in the case ofusing octylamine to coordinate sulfur.

Second Semiconductor Materials—Single-Source Precursors

For a compound semiconductor nanoparticle comprising of elements ZnS orCd_(x)Zn_((1-x))S a source for Zn or Cd and S source can also be in thefrom of a single-source precursor, whereby the precursor to be usedcontains both Zn or Cd and S within the single molecule. This precursorcan be an organometallic compound and inorganic salt or a coordinationcompound, (Zn_(a)S_(b))L_(c) or (Cd_(x)Zn_((1-x))S)_(n)L_(m) Where Zn orCd and S are the elements required within the nanoparticles and L is thecapping ligands.

Examples for an II-VI semiconductor were M=II and E=VI element can bebut is not restricted to bis(dialkyldithio-carbamato)M, (II) complexescompounds of the formula M(S₂CNR₂)₂ M=Zn, Cd; S═S, and R=alkyl or arygroups; CdS Cd[SSiMe₃]₂, Cd(SCNHNH₂)₂Cl₂, Cd(SOCR)₂.py; [RME^(t)Bu]₅M=Zn, Cd; E=S; R=Me, Et, Ph: [X]₄[E₄M₁₀(SR)₁₆] E=S, M=Zn, Cd; X=Me₃NH⁺,Li⁺, Et₃NH⁺: [Cd₃₂S₁₄(SPh)₃₆].L:

[M₄(SPh)₁₂]⁺[X]₂ ⁻M=Zn, Cd; X=Me₄N⁺, Li⁺: [Zn(SEt)Et]₁₀: [MeMe^(i)Pr]M=Zn, Cd; E=S: [RCdSR′]₅ R═O(ClO₃), R′═PPh₃, ^(i)Pr:[Cd₁₀S₄(S′Ph)₁₂(PR₃)₄]

Detailed Discussion

The synthesis of quantum dot-quantum wells is preferably a three-stepprocess, optionally involving isolation of the product of a step priorto further modification to provide the next layer of the nanoparticlestructure. By way of example, for the nanoparticle,ZnS/CdSe/Cd_(x)Zn_(1-x)S, the cores are synthesized and isolated from agrowth solution and the first shell is grown onto the cores in aseparate reaction and isolated once again. Finally an outerCd_(x)Zn_(1-x)S shell layer is grown onto the core-shell structure toproduce the ZnS/CdSe/Cd_(x)Zn_(1-x)S quantum dot-quantum well.

Synthesis of ZnS Cores

Zinc sulfide (or cadmium/zinc sulphide) particles were synthesized by anumber of methods when a small quantity was needed by decomposing[Et₃NH]₄[Zn₁₀S₄(SPh)₁₆] clusters in HDA at 180° C. and heating to 250°C. or 300° C. to produce 2 nm or 5.5 nm diameter ZnS particles.

Synthesis of ZnS/CdSe Core Shell Dots

Either a combination of two precursors was used such as Me₂Cd and TOPSeor a single-source precursor such as [Et₃NH]₄[Cd₁₀Se₄(SPh)₁₆] was usedas precursors for the formation of the CdSe layer. The precursorsdecompose onto the ZnS cores enabled the synthesis of multi-gramquantities of ZnS/CdSe core-shell particles.

Quantum Well Modifications ZnS/CdSe/Cd_(x)Zn_(1-x)S

Growth of the Cd_(x)Zn_(1-x)S shell is performed at a low temperatureand added very slowly to prevent thick shell growth and renucleation ofCdSe nanoparticles. The likelihood of alloying is minimal at this growthtemperature. The ZnS/CdSe core-shell nanocrystals exhibit quantumefficiencies of about 3%. The growth of the outer Cd_(x)Zn_(1-x)S alsoshifts the emission and the first absorption feature by about 2 nm.Again, similar shifts in the emission/absorption are common with CdSe orCdS overcoated with ZnS.

Cadmium-Free Quantum Dot-Quantum Wells

There is also a great need for quantum dots that perform similarly toCdSe—ZnS core-shell quantum or quantum dot-quantum wells that arecadmium free. Nanoparticles in accordance with the present invention maytherefore be produced which include a layer of cadmium-freesemiconductor material in place of a cadmium-containing layer. Forexample, the nanoparticles ZnS/CuInS₂/ZnS and ZnS/CuInSe₂/ZnS can beproduced in accordance with the method of the present invention and usedin place of ZnS/CdS/ZnS and ZnS/CdSe/ZnS.

ZnS/CuInS₂ Core/Shell Structure

This was achieved by using either a combination of precursors eachcontaining just one element required within the final compositenanoparticle or by the use of single-source precursors which contain allor more than one element required within the final composite.

Multi-Source Precursors

ZnS core particles were dissolved in warm capping agent/solvent such asHDA-hexanethiol or TOPO-hexanethiol followed by the addition of a coppersource, an indium source and a sulfur source such as CuI dissolved in anamine, InI₃ dissolved in an amine and sulfur coordinated to TOP to giveTOPS. The growth of the CuInS₂ shell onto the ZnS cores is achieved bythe addition of the above precursors to the HDA-hexanethiol solutionwhile increasing the temperature between 150° and 300° C. The solutionwas then cooled to 150° C. before further precursor additions, thisbeing repeated until the desired emission wavelength was achieved. Theparticles-containing solution was then cooled and the particles isolatedusing excess methanol.

Single-Source Precursors

Single-source precursors may be used such as (Ph₃P)₂CuIn(SEt)₄ or acombination of single-source precursors such as In(S₂CNEt₂)₃ andCu(S₂CNEt₂)₂.

ZnS/CuInSe₂ Core/Shell Structure

This was achieved by using either a combination of precursors eachcontaining just one element required within the final compositenanoparticle or by the use of single-source precursors which contain allor more than one element required within the final composite.

Multi-Source Precursors

ZnS core particles were dissolved in warm capping agent/solvent such asHDA or TOPO-hexanethiol mix followed by the addition of a copper sourcean indium source and a selenium source such as CuI dissolved in anamine, InI₃ dissolved in an amine and selenium coordinated to TOP togive TOPSe. The growth of the CuInSe₂ shell onto the ZnS cores isachieved by the addition of the above precursors to the HDA-hexanethiolsolution while increasing the temperature between 150° and 300° C. Thesolution was then cooled to 150° C. before further additions, this beingrepeated until the desired emission wavelength was achieved. Theparticles containing solution was then cooled and the particles isolatedusing excess methanol.

Single-Source Precursors

Single-source precursors may be used such as (Ph₃P)₂CuIn(SeEt)₄ or acombination of single-source precursors such as In(Se₂CNEt₂)₃ andCu(Se₂CNEt₂)₂.

ZnS/CuInS₂/ZnS and ZnS/CuInSe₂/ZnS Core/Multishell Nanoparticles

The amount of zinc and sulfur precursor used was varied depending on thethickness of the outer ZnS shell required. ZnS/CuInS₂ or ZnS/CuInSe₂particles were added to degassed HDA at 70° C. and heated to 180-200° C.Me₂Zn and sulfur solutions were used to grow the outer ZnS layers bydropwise addition until the desired ZnS shell thickness was reached.

By the use of an in situ optical probe, moreover, an Ocean OpticsUSB2000 spectrometer, the progressive formation/growth of the core,core-shell or quantum-well particle can be followed by the maximum ofthe photoluminescence emission peak or the maximum of the absorptionspectra, when the required the photoluminescence emission was achievedthe reaction was stopped by cooling the reaction solution.

The present invention is illustrated with reference to the followingfigures and non-limiting Example and Reference Examples, in which:

FIG. 1 is an illustration of a) Core nano-particle comprising of a ZnScore and HDA as an organic capping agent, b) core-shell particlecomprising of a ZnS core a CdSe shell and HDA as an organic cappingagent, c) quantum dot-quantum well organic capped particle comprising ofa ZnS core a CdSe shell followed by a Cd_(x)Zn_(1-x)S shell with a HDAcapping agent;

FIG. 2 is an illustration of a) Core nano-particle comprising of a ZnScore and HDA as an organic capping agent, b) core-shell particlecomprising of a ZnS core a CdSe shell and HDA as an organic cappingagent, c) quantum dot-quantum well organic capped particle comprising ofa ZnS core a CdSe shell followed by a ZnS shell with a HDA capping agentd) quantum dot-multi quantum well comprising of a ZnS core a CdSe shellfollowed by a shell of CdS followed by another shell of ZnS with a HDAcapping agent;

FIG. 3 is a diagram of a) core particle comprising of a ZnS core and HDAas an organic capping agent, b) core-shell particle comprising of a ZnScore a CuInS₂ shell and HDA as an organic capping agent, c) quantumdot-quantum well organic capped particle comprising of a ZnS core aCuInS₂ central layer followed by a ZnS shell with a HDA capping agent;

FIG. 4 illustrates properties of ZnS core quantum dots a) excitation (tothe left) and emission spectra of 5.5 nm ZnS nanocrystals. (b) Powderx-Ray diffraction pattern of 5.5 nm. (c) Transmission electronmicrograph (TEM) image of 5.5 nm ZnS core. Inset shows a high-resolutionimage of a single ZnS particle;

FIG. 5 shows absorption and photoluminescence spectra for a core-shellZnS—CdSe quantum dots with an outer capping layer of hexadecylamine(HDA), with the absorption maximum at 440 nm and the emission maximum at460 nm;

FIG. 6 shows absorption and PL spectra of ZnS/CdSe/Cd_(x)Zn_((1-x))Squantum well nanocrystals. The longest wavelength adsorption featureoccurs at λ=453 nm and the maximum emission peak is at λ=472 nm;

FIG. 7 shows absorption and PL spectra of ZnS cores;

FIG. 8 shows absorption and PL spectra of ZnSe cores;

FIGS. 9A and 9B show absorption and PL spectra of ZnS/InP core/shellnanocrystals respectively;

FIGS. 10A and 10B show absorption and PL spectra of ZnS/InP core/shellnanocrystals respectively in which the ZnS cores are larger than thoseshown in FIGS. 9A and 9B;

FIGS. 11A and 11B show PL and absorption spectra of ZnS/InP/ZnS quantumwell nanocrystals; and

FIG. 12 shows a PL spectrum for the growth of ZnSe quantum dots.

EXAMPLES

All syntheses and manipulations were carried out under a dry oxygen-freeargon or nitrogen atmosphere using standard Schlenk or glove boxtechniques. All solvents were distilled from appropriate drying agentsprior to use (Na/K-benzophenone for THF, Et₂O, toluene, hexanes andpentane). HDA, octylamine, hexanethiol, dioctylphalate, TOP,Cd(CH₃CO₂)₂, sulfur, selenium powder, CdO₂, CdCO₃, InI, CuI (Adrich)were procured commercially and used without further purification.

UV-vis absorption spectra were measured on a Heλiosβ Thermospectronic.Photoluminescence (PL) spectra were measured with a Fluorolog-3 (FL3-22)photospectrometer at the excitation wavelength 380 nm. Powder X-Raydiffraction (PXRD) measurements were preformed on a Bruker AXS D8diffractometer using monochromated Cu-K_(α) radiation.

Cluster Preparation Preparation of [HNEt₃]₂[Zn₄(SPh)₁₀]

To a stirred methanol (360 ml) solution of benzenethiol (168 ml, 1.636mmol) and triethylamine (229 ml, 1.64 mmol) was added dropwiseZn(NO₃)₂.6H₂O (189 g, 0.635 mol) that had previously been dissolved inmethanol (630 ml). The solution was then allowed to stir while warminguntil the precipitate had completely dissolved to leave a clearsolution. This was then place at 5° C. for 24 h in which time largecolourless crystals of [HNEt₃]₂[Zn₄(SPh)₁₀] had formed (169 g).

Preparation of [HNEt₃]₄[Zn₁₀S₄(SPh)₁₆]

To a stirred acetonitrile (100 ml) solution of [HNEt₃]₂[Zn₄(SPh)₁₀](168.9 g, 0.1086 mol) was added 3.47 g (0.1084 mmol) of sulfur powder,the resulting slurry was left to stirrer for 10 minutes. A further 750ml of acetonitrile was added and the solution warmed to 75° C. to give aclear pale yellow solution which was allowed to cool to 5° C., yieldinglarge colourless crystals (74.5 g). The crystals were washed in hexaneto give 71.3 g of [HNEt₃]₄[Zn₁₀S₄(SPh)₁₆].

Preparation of Quantum Dot Cores (ZnS or Cd_(x)Zn_((1-x))S)

Method 1—Preparation of ZnS Nanoparticles from[Et₃NH]₄[Zn₁₀S₄(SPh)₁₆]/TOPS/Me₂Zn in HDA by Dropwise Addition ofMe₂Zn.TOP

HDA was placed in a three-neck round bottomed flask and dried anddegassed by heating to 120° C. under a dynamic vacuum for >1 hour. Thesolution was then cooled to 60° C. To this was added[HNEt₃]₄[Zn₁₀S₄(SPh)₁₆]. Initially 4 mmol of TOPS and 4 mmols ofMe₂Zn.TOP were added to the reaction at room temperature and thetemperature increased and allowed to stir for 2 hours. The temperaturewas progressively increased at a rate of ˜1° C./5 min with equimolaramounts of TOPS and Me₂Zn.TOP being added dropwise as the temperaturewas steadily increased. The reaction was stopped when the PL emissionmaximum had reached the required emission, by cooling to 60° C. followedby addition of 300 ml of dry ethanol or acetone. This produced wasisolated by filtration. The resulting ZnS particles which wererecrystallized by re-dissolving in toluene followed by filtering throughCelite followed by re-precipitation from warm ethanol to remove anyexcess HDA, selenium or cadmium present.

Method 2 (for Reference Purposes Only)

2 nm cores were prepared in 250 g hexadecylamine (HDA) which waspreviously degassed at 120° C. for one hour then, under nitrogen,[Et₃NH]₄[Zn₁₀S₄(SPh)₁₆] (4.75 g, 1.64 mmol) was added and the solutionwas heated to 250° C. for 30 minutes which resulted in the nucleationand growth of ZnS nanoparticles. The resulting solution was then cooledto 65° C. and the particles were isolated by the addition of 400 ml drymethanol giving 1.1 g ZnS particles with approximately 20% w/w of ZnS.To grow 5.5 nm ZnS, the above-mentioned procedure was repeated at 300°C. growth temperature for 30 minutes giving 0.69 g ZnS particles withapproximately 33% w/w of ZnS.

Synthesis of ZnS/CdSe Composite Quantum Dots Method 1

In a typical synthesis, 0.35 g ZnS cores (or approximately 4.9×10⁷particles) were added to 100 g of degassed HDA at 70° C., the solutionwas then heated to 150° C. The growth of the CdSe layer onto the ZnScore is achieved by a successive addition of the cluster[Et₃NH]₄[Cd₁₀Se₄(SPh)₁₆] to the ZnS-HDA solution, between 150 to 300° C.for two hours. The solution was cooled to 150° C. before the furtheraddition of precursor. The ZnS/CdSe particles were then cooled andisolated with excess methanol.

Method 2

In a typical synthesis, ZnS cores were added to degassed andmoisture-free HDA at 70° C., the solution was then heated to 150° C. Thegrowth of the CdSe layer onto the ZnS core is achieved by a successiveaddition of Me₂Cd.TOP and TOPSe to the ZnS-HDA solution, between 150 to300° C. for two hours. The solution was then cooled to 150° C. beforeadditional Me₂Zn′TOP and TOPS were added, this was repeated until thedesired emission wavelength was achieved.

Synthesis of ZnS/CdSe/Cd_(x)Zn_(1-x)S

The amount of zinc, cadmium and sulfur precursor used was varieddepending on the thickness of the outer Cd_(x)Zn_(1-x)S shell required.The synthesis of ZnS/CdSe/Cd_(x)Zn_(1-x)S 2.5 ml Me₂Cd (0.05M), 2.5 mlMe₂Zn (0.05M) solutions along with 5.0 ml 0.05M sulfur solution wasadded to the ZnS/CdSe cores to produce ZnS/CdSe/Cd_(x)Zn_(1-x)Snanoparticles.

Reference Examples Preparation of ZnS/InP/ZnS and ZnSe/InP/ZnSe QuantumDot-Quantum Wells Preparation of Core ZnS

HDA(250 g) was placed in a three neck flask and degassed at 120° C.under vacuum for one hour. At 100° C. [Et₃NH₄][Zn₁₀S₄(SPh)₁₆] (10 g) wasadded and the solution was then heated to 300° C. for 30 minutes. After30 minutes, the solution was cooled to 200° C. and the reaction mixturewas annealed for one hour. The reaction mixture was left to coolovernight to room temperature.

Particles of HDA coated ZnS were isolated by the addition of warm drymethanol (250 ml). The precipitation of white particles occurred thesewere isolated by centrifugation, washed with acetone and left to dryunder nitrogen. Mass of product=1.7442 g. UV-vis and PL spectra of theZnS cores are shown in FIG. 7.

Preparation of Core ZnSe

HDA(150 g) was placed in a three neck flask, dried and degassed at 120°C. for one hour. After one hour the mixture was cooled to 60° C.[Zn₁₀Se₄(SPh)₁₆][Et₃NH₄] (5 g) was added to the HDA under nitrogen at90° C. and left to stir for 5 mins before adding TOPSe (3.53 ml). Thereaction mixture changed colour from colorless to pale yellow. Thetemperature was increased to 120° C. The temperature of the reactionmixture was then increased gradually to 280° C. After 280° C. thereaction was left to cool. Once the temperature had decreased to 65° C.,the particles were isolated by addition of methanol (250 ml) followed bycentrifuged, washed with acetone and left to dry under nitrogen. Mass ofproduct=1.2443 g. UV-vis and PL spectra of the ZnSe cores are shown inFIG. 8.

Preparation of Core-Shell ZnS/InP Method (a)

Dibutyl ester (50 ml) and stearic acid (5.65 g) were dried/degassed byheating to between 65-100° C. under vacuum for 1 hour. The temperaturewas then increased to 180° C. followed by the addition of InMe₃ (1.125ml), (TMS)₃P (1.125 ml) and ZnS particles (0.235 g) and left to stir for10 mins. The reaction mixture turned pale yellow after 5 mins ofaddition. When the reaction temperature had reached 200° C., furtherquantities of InMe₃ (2.25 ml) and (TMS)₃P (2.25 ml) were added dropwisewhich resulted in the colour changing from pale yellow to clear brightorange, the temperature was subsequently increased to 220° C. This wasfollowed by further addition of InMe₃ (3.375 ml) and (TMS)₃P (3.375 ml)resulting in a dark red solution colour.

The reaction mixture was then left to anneal for 1 hour at 220° C. andthen allowed to cool to room temperature. This was followed byisolation; by adding 100 ml of dry warm ethanol which produced aprecipitate of orange/red particles which were isolated viacentrifugation, washed with acetone and left to dry. Mass ofproduct=2.29 g. UV-vis spectrum of the ZnS/InP core/shell particles isshown in FIG. 9A. PL spectrum of the ZnS/InP core/shell particles isshown in FIG. 9B.

Preparation of Core-Shell ZnS/InP Method (b) (Using Larger Sized ZnSCore Particles)

Dibutyl ester (50 ml) and stearic acid (5.65 g) were dried/degassed byheating to between 65-100° C. under vacuum for 1 hour. The temperaturewas then increased to 180° C. and ZnS particles (0.5 g) along with InMe₃(1.125 ml) and (TMS)₃P (1.125 ml) were added dropwise under N₂ to thereaction solution this was left to stir for 10 mins, in which time thereaction mixture turned pale yellow. When the reaction temperature hadreached 200° C., further addition of InMe₃ (2.25 ml) and (TMS)₃P (2.25ml) was made which resulted in the colour changing from pale yellow toclear bright orange. The temperature was then increased to 220° C., withfurther addition of InMe₃ (3.375 ml) and (TMS)₃P (3.375 ml) resulting inthe reaction solution turning a dark red solution colour.

The reaction mixture was then left to anneal for 1 hour at 220° C.followed by cooling to room temperature. 100 ml of dry warm ethanol wasthen added to gave a precipitate of orange/red particles, theseparticles were isolated by centrifugation, washed with acetone and leftto dry. Mass of product=3.2844 g. UV-vis spectrum of the ZnS/InPcore/shell particles is shown in FIG. 10A. PL spectrum of the ZnS/InPcore/shell particles is shown in FIG. 10B.

Preparation of Core-Shell ZnSe/InP

Dibutyl ester (50 ml) and stearic acid (5.65 g) were placed in a threeneck flask and dried and degassed for one hour at a temperature of 90°C. The temperature was increased to 180° C. with addition of ZnSeparticles (0.5 g), (TMS)₃P (1.125 ml) and InMe₃ (1.125 ml). The solutionwas left at 180° C. for 10 mins followed by increasing the temperatureto 200° C. At 200° C. a further addition of (TMS)₃P (2.25 ml) and InMe₃(2.25 ml) was made. The temperature was then increased to 220° C.followed by a final addition of (TMS)₃P (3.375 ml) and InMe₃ (3.375 ml).The reaction mixture changed colour from orange/yellow to dark red andwas left to anneal for one hour at 220° C. before cooling to roomtemperature. 100 ml of dry warm ethanol was then added to the reactionsolution to give a precipitate of orange/red particles, which wereisolated by centrifugation, washed with acetone and left to dry. Mass ofproduct=3.33 g.

Final Shelling Preparation of ZnS/InP/ZnS

HDA (150 g) was placed in a 3 neck flask and dried and degassed for onehour the temperature was then increased to 200° C. In a separate flaskcore-shell particles of ZnS/InP (with an orange emission) (2.6343 g)were dissolved in Dibutyl ester (5 ml) and placed under vacuum for 20mins this was followed by sonication for 5 mins, this was followed bythe addition of (TMS)₃S (3.75 ml). This solution was then added to theHDA solution dropwise followed by the addition of Zn(Et₂) dissolved TOP(7.50 ml). The reaction mixture was left at 200° C. for 26 hours. After26 hours some luminescence was observed. The temperature was thendecreased to room temperature followed by the addition of chloroform.The reaction solution was then filtered through Celite. The QD-QW's werethen isolated under nitrogen by addition of warm dry methanol followedby centrifugation. UV-vis spectrum of the ZnS/InP/ZnS core/shell/shellparticles is shown in FIG. 11A. PL spectrum of the ZnS/InP/ZnScore/shell/shell particles is shown in FIG. 11B.

Preparation of ZnSe Quantum Dots

Alternative methods are set out below for preparing ZnSe quantum dotswhich can be further modified for use as cores in the preparation ofcore/multishell quantum dot-quantum wells as described above.

Molecular Cluster Method

[Et₃NH]₄[Zn₁₀Se₄(SPh)₁₆] (2.5 g) and 5 mmol TOP-Se were added to astirred solution of HDA (55 g) under N₂ while at 100° C. using standardairless techniques. The temperature was then increased to 250° C. thiswas left to stir for 2 hours, the initial PL peak of ZnSe was at 385 nm.Zn(Et)₂ and further quantities of and TOP-Se precursors were added tothe reaction solution while the temperature was slowly increased to 290°C. Further quantities of Zn(Et)₂ and TOP-Se were added while thetemperature was kept at 290° C. The growth of ZnSe was followed bymonitoring the evolution of UV-Vis absorption and PL emission.

1. 1 ml TOP-Se (0.5M) and 1 ml Zn(Et)₂ (0.5M) was slowly injected intothe above reaction solution at 290° C., and then kept at 290° C. for 30mins. The obtained PL is 393 nm.

2. 2 ml TOP-Se (0.5M) and 2 ml Zn(Et)₂ (0.5M) was added into thereaction solution at 290° C. and then kept at 290° C. for 60 mins. Theobtained PL is 403 nm.

3. Additional of 2 ml, 2 ml, 3 ml and 3 ml etc of the same stocksolution was dropwise injected into reaction solution by the samereaction condition.

4. The PL peak will be the red-shift with the multi-injection of Zn(Et)₂and TOP-Se precursors and the longer annealing time. The maximum finialPL peak can reach to 435 nm (See FIG. 12).

5. Total 20 mmol TOP-Se and Zn(Et)₂ were used to make ZnSenanoparticles.

6. The final ZnSe nanoparticle was collected by size selectiveprecipitation with hot butanol (70° C.), centrifugation and thenredispersed in octane. Excess HDA was completely removed by repeatingthose previous steps. The particles were re-dispersed in toluene,hexane, heptane and octane, resulting in clear nanoparticle solution.

The PL peak width of ZnSe product by this method is as narrow as 16 nmwith a QY of 1020%.

Preparation of ZnSe Quantum Dots Dual Source Precursor Method

ZnSe quantum dots were prepared by using the injection of 5 ml Zn(Et)₂(0.5M) and 5 ml TOP-Se (0.5M) into ODA at 345° C.

After obtaining the ZnSe quantum dots, the multi-injection of Zn(Et)₂and TOP-Se precursors for the growth of larger ZnSe nanoparticles wasanalogous to the above Cluster Method for the production of ZnSe quantumdots.

The PL peak width of ZnSe product by this method is as narrow as 20 nmwith a QY of 10˜30%.

1. A nanoparticle comprising a core comprising a core semiconductormaterial, a first layer comprising a first semiconductor materialprovided on said core and a second layer comprising a secondsemiconductor material provided on said first layer, said coresemiconductor material being different to said first semiconductormaterial and said first semiconductor material being different to saidsecond semiconductor material, wherein: a) at least two of the core,first shell and second shell materials incorporate ions from groups 12and 15 of the periodic table, groups 14 and 16 of the periodic table, orgroups 11, 13 and 16 of the periodic table; or b) the second shellmaterial incorporates ions of at least two different elements from group12 of the periodic table and ions from group 16 of the periodic table;or c) at least one of the core, first and second semiconductor materialsincorporates ions from groups 11, 13 and 16 of the periodic table and atleast one other of the core, first and second semiconductor materials isa semiconductor material not incorporating ions from groups 11, 13 and16 of the periodic table.
 2. A nanoparticle according to claim 1,wherein: at least two of the core, first shell and second shellmaterials incorporate ions from groups 12 and 15 of the periodic table,groups 14 and 16 of the periodic table, or groups 11, 13 and 16 of theperiodic table; and the other of the core, first and secondsemiconductor materials incorporates ions from the group consisting of:groups 12 and 15 of the periodic table, groups 13 and 15 of the periodictable, groups 12 and 16 of the periodic table, groups 14 and 16 of theperiodic table, and groups 11, 13 and 16 of the periodic table.
 3. Ananoparticle according to claim 1, wherein: the second shell materialincorporates ions of at least two different elements from group 12 ofthe periodic table and ions from group 16 of the periodic table; andsaid second semiconductor material has the formula M_(x)N_(1-x)E, whereM and N are the group 12 ions, E is the group 16 ion, and 0<x<1.
 4. Ananoparticle according to claim 3, wherein 0.1<x<0.9.
 5. A nanoparticleaccording to claim 1, wherein: at least one of the core, first andsecond semiconductor materials incorporates ions from groups 11, 13 and16 of the periodic table and at least one other of the core, first andsecond semiconductor materials is a semiconductor material notincorporating ions from groups 11, 13 and 16 of the periodic table; andsaid at least one other of the core, first and second semiconductormaterials not incorporating ions from groups 11, 13 and 16 of theperiodic table incorporates ions from the group consisting of: groups 12and 15 of the periodic table, groups 13 and 15 of the periodic table,groups 12 and 16 of the periodic table, and groups 14 and 16 of theperiodic table.
 6. A nanoparticle according to claim 1, wherein thenanoparticle further comprises a third layer of a third semiconductormaterial provided on said second layer.
 7. A nanoparticle according toclaim 6, wherein said third semiconductor material is selected from thegroup consisting of: a semiconductor material incorporating ions fromgroups 12 and 15 of the periodic table, a semiconductor materialincorporating ions from groups 13 and 15 of the periodic table, asemiconductor material incorporating ions from groups 12 and 16 of theperiodic table, a semiconductor material incorporating ions from groups14 and 16 of the periodic table and a semiconductor materialincorporating ions from groups 11, 13 and 16 of the periodic table.
 8. Ananoparticle according to claim 1, wherein the group 12 ions areselected from the group consisting of: zinc ions, cadmium ions, andmercury ions.
 9. A nanoparticle according to claim 1, wherein the group15 ions are selected from the group consisting of: nitride ions,phosphide ions, arsenide ions, and antimonide ions.
 10. A nanoparticleaccording to claim 1, wherein the group 14 ions are selected from thegroup consisting of: lead ions, tin ions, and germanium ions.
 11. Ananoparticle according to claim 1, wherein the group 16 ions areselected from the group consisting of: sulfide ions, selenide ions, andtelluride ions.
 12. A nanoparticle according to claim 1, wherein thegroup 11 ions are selected from the group consisting of: copper ions,silver ions, and gold ions.
 13. A nanoparticle according to claim 1,wherein the group 13 ions are selected from the group consisting of:aluminium ions, indium ions, and gallium ions.
 14. A method forproducing a nanoparticle comprising a core comprising a coresemiconductor material, a first layer comprising a first semiconductormaterial provided on said core and a second layer comprising a secondsemiconductor material provided on said first layer, said coresemiconductor material being different to said first semiconductormaterial and said first semiconductor material being different to saidsecond semiconductor material, wherein: a) at least two of the core,first shell and second shell materials incorporate ions from groups 12and 15 of the periodic table, groups 14 and 16 of the periodic table, orgroups 11, 13 and 16 of the periodic table; or b) the second shellmaterial incorporates ions of at least two different elements from group12 of the periodic table and ions from group 16 of the periodic table;or c) at least one of the core, first and second semiconductor materialsincorporates ions from groups 11, 13 and 16 of the periodic table and atleast one other of the core, first and second semiconductor materials isa semiconductor material not incorporating ions from groups 11, 13 and16 of the periodic table, the method comprising effecting conversion ofa nanoparticle core precursor composition to the material of thenanoparticle core, depositing said first layer on said core anddepositing said second layer on said first layer.
 15. A method accordingto claim 14, wherein said nanoparticle core precursor compositioncomprises first and second core precursor species containing the ions tobe incorporated into the growing nanoparticle core.
 16. A methodaccording to claim 15, wherein said first and second core precursorspecies are separate entities contained in said core precursorcomposition, and said conversion is effected in the presence of amolecular cluster compound under conditions permitting seeding andgrowth of the nanoparticle core.
 17. A method according to claim 15,wherein said first and second core precursor species are combined in asingle entity contained in said core precursor composition.
 18. A methodaccording to claim 14, wherein conversion of the core precursorcomposition to the nanoparticle core is effected in a reaction mediumand said nanoparticle core is isolated from said reaction medium priorto deposition of the first layer.
 19. A method according to claim 14,wherein deposition of said first layer comprises effecting conversion ofa first semiconductor material precursor composition to said firstsemiconductor material.
 20. A method according to claim 19, wherein saidfirst semiconductor material precursor composition comprises third andfourth precursor species containing the ions to be incorporated into thegrowing first layer of the nanoparticle.
 21. A method according to claim20, wherein said third and fourth precursor species are separateentities contained in said first semiconductor material precursorcomposition.
 22. A method according to claim 20, wherein said third andfourth precursor species are combined in a single entity contained insaid first semiconductor material precursor composition.
 23. A methodaccording to claim 14, wherein deposition of said second layer compriseseffecting conversion of a second semiconductor material precursorcomposition to said second semiconductor material.
 24. A methodaccording to claim 23, wherein said second semiconductor materialprecursor composition comprises fifth and sixth precursor speciescontaining the ions to be incorporated into the growing second layer ofthe nanoparticle.
 25. A method according to claim 24, wherein said fifthand sixth precursor species are separate entities contained in saidsecond semiconductor material precursor composition.
 26. A methodaccording to claim 24, wherein said fifth and sixth precursor speciesare combined in a single entity contained in said second semiconductormaterial precursor composition.
 27. A nanoparticle comprising a corecomprising a core semiconductor material, a first layer comprising afirst semiconductor material provided on said core and a second layercomprising a second semiconductor material provided on said first layer,said core semiconductor material being different to said firstsemiconductor material and said first semiconductor material beingdifferent to said second semiconductor material, said nanoparticleproduced according to a method comprising: effecting conversion of ananoparticle core precursor composition to the material of thenanoparticle core, depositing said first layer on said core anddepositing said second layer on said first layer, said core precursorcomposition comprising a first precursor species containing a first ionto be incorporated into the growing nanoparticle core and a separatesecond precursor species containing a second ion to be incorporated intothe growing nanoparticle core, said conversion being effected in thepresence of a molecular cluster compound under conditions permittingseeding and growth of the nanoparticle core.
 28. A nanoparticleaccording to claim 27, wherein: a. at least two of the core, first shelland second shell materials incorporate ions from groups 12 and 15 of theperiodic table, groups 14 and 16 of the periodic table, or groups 11, 13and 16 of the periodic table; or b. the second shell materialincorporates ions of at least two different elements from group 12 ofthe periodic table and ions from group 16 of the periodic table; or c.at least one of the core, first and second semiconductor materialsincorporates ions from groups 11, 13 and 16 of the periodic table and atleast one other of the core, first and second semiconductor materials isa semiconductor material not incorporating ions from groups 11, 13 and16 of the periodic table.